Would anyone find assembly tuts useful under linux on the raspberry pi.
If so what examples would you be interested in seeing examples of ?.
Re: Asm tuts
I would love to see some. I still have this book on my things to get list http://www.amazon.com/Raspberry-Pi-Asse ... 148112790X I think maybe some easy games or anything could be helpful.
http://www.raspians.com - always looking for content feel free to ask to have it posted. Or sign up and message me to become a contributor to the site. Raspians is not affiliated with the Raspberry Pi Foundation. (RPi's + You = Raspians)
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antiloquax
- Posts: 406
- Joined: Sun Nov 20, 2011 11:37 am
- Contact: Website
Re: Asm tuts
Hi DexOS,
I'd love some assembly tutorials.
I'd love some assembly tutorials.
-
antiloquax
- Posts: 406
- Joined: Sun Nov 20, 2011 11:37 am
- Contact: Website
Re: Asm tuts
I am particulaly keen to learn the system calls to get input from the keyboard and write out to the screen, eg to take in some numbers, apply a function and print the result. I know how to do this by calling printf, but that's not pure asm.
Re: Asm tuts
that's for riscos so won't do you much good.malakai wrote:I would love to see some. I still have this book on my things to get list http://www.amazon.com/Raspberry-Pi-Asse ... 148112790X I think maybe some easy games or anything could be helpful.
Re: Asm tuts
Why not? The underlying assembly language is exactly the same between RISCOS and Linux (and anything else on the Raspi) so all the principles apply. And it's so easy to move to and from OS's on the Raspi..sej7278 wrote:that's for riscos so won't do you much good.malakai wrote:I would love to see some. I still have this book on my things to get list http://www.amazon.com/Raspberry-Pi-Asse ... 148112790X I think maybe some easy games or anything could be helpful.
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Contrary to popular belief, humorous signatures are allowed.
I've been saying "Mucho" to my Spanish friend a lot more lately. It means a lot to him.
Contrary to popular belief, humorous signatures are allowed.
I've been saying "Mucho" to my Spanish friend a lot more lately. It means a lot to him.
Re: Asm tuts
I was going to write some tuts, but when i saw just one reply, i thought theres no demand.
So i will write some tuts, but for now to give you a taste, of asm coding for linux arm, heres some examples for the pi and linux.
But first note, i use a assembler called FasmArm, which does not have a Arm port (i am working on this), so for now you need to assembly theres examples on windows or x86 linux.
Once you have assembled the code, you will need to right click on the file, then click "properties", them "permissions" and click the "Allow executing as program"
Then you can run them on the pi from a terminal like this:
./filename <enter>
or some may need
sudo /filename <enter>
Replace filename with the name of file.
First the classic hello world
In the above example i have included the x86 assembly code to compare.
The "ArmMacro.inc" needed for the above
Same as above but change text color
Get user input
And here a very small full working http server
I will post using frame buffer and maping the gpio and some games etc.
So i will write some tuts, but for now to give you a taste, of asm coding for linux arm, heres some examples for the pi and linux.
But first note, i use a assembler called FasmArm, which does not have a Arm port (i am working on this), so for now you need to assembly theres examples on windows or x86 linux.
Once you have assembled the code, you will need to right click on the file, then click "properties", them "permissions" and click the "Allow executing as program"
Then you can run them on the pi from a terminal like this:
./filename <enter>
or some may need
sudo /filename <enter>
Replace filename with the name of file.
First the classic hello world
Code: Select all
format ELF executable
entry start
include 'ArmMacro.inc'
segment readable writeable executable
start: ; (x86 code)
;
mov r0, 0x1 ; fd -> stdout (mov ebx,1)
load_reg r1, msg ; buf -> msg (mov ecx,msg)
load_reg r2, msg_size ; count -> len(msg) (mov edx,msg_size)
mov r7, 0x4 ; write is syscall (mov eax,4
swi 0x0 ; invoke syscall (int 80h)
;
mov r0, 0x0 ; status -> 0 (xor ebx,ebx)
mov r7, 0x1 ; exit is syscall 1 (mov eax,1)
swi 0x0 ; invoke syscall (int 80h)
;
segment readable writeable ;
;
msg db "Hello from FasmArm, on the Raspberry Pi ",0xa ; Message
msg_size = $-msg
The "ArmMacro.inc" needed for the above
Code: Select all
; ----------------------------------------------------- ;
; Use load_reg Rd,$12345678 in your code etc... ;
; ----------------------------------------------------- ;
macro load_reg reg,immediate { ;
mov reg,immediate and $FF ;
orr reg,reg,immediate and $FF00 ;
orr reg,reg,immediate and $FF0000 ;
orr reg,reg,immediate and $FF000000 ;
} ;
Code: Select all
; Same as print, but changes color of text
format ELF executable
entry start
include 'ArmMacro.inc'
segment readable writeable executable
start:
;
mov r0, 0x1 ; fd -> stdout (mov ebx,1)
load_reg r1, msg ; buf -> msg (mov ecx,msg)
load_reg r2, msg_size ; count -> len(msg) (mov edx,msg_size)
mov r7, 0x4 ; write is syscall (mov eax,4
swi 0x0 ; invoke syscall (int 80h)
;
mov r0, 0x0 ; status -> 0 (xor ebx,ebx)
mov r7, 0x1 ; exit is syscall 1 (mov eax,1)
swi 0x0 ; invoke syscall (int 80h)
;
segment readable writeable
msg db 1bh, "[0;31;49m Your message", 1bh, "[0;39;49m",0xa ;1bh, "[0;31;49m = red on black
msg_size = $-msg ;1bh, "[0;39;49m" =restore default colorCode: Select all
; Get user input
format ELF executable
entry start
include 'ArmMacro.inc'
segment readable writeable executable
start: ;
mov r0, 0x1 ;
load_reg r1, msg ;
load_reg r2, msg_size ;
mov r7, 0x4 ;
swi 0x0 ;
;
mov r7, 0x3 ;
mov r0, 0x1 ;
load_reg r1, inp_buf ;
load_reg r2, 0x100 ;
swi 0x0 ;
;
load_reg r1, inp_buf_size ;
str r0, [r1] ;
;
mov r0, 0x1 ;
load_reg r1, msg2 ;
load_reg r2, msg2_size ;
mov r7, 0x4 ;
swi 0x0 ;
;
mov r0, 0x1 ;
load_reg r1, inp_buf ;
load_reg r2, inp_buf_size ;
ldr r2, [r2] ;
mov r7, 0x4 ;
swi 0x0 ;
;
mov r0, 0x1 ;
load_reg r1, crlf ;
load_reg r2, 0x1 ;
mov r7, 0x4 ;
swi 0x0 ;
;
mov r0, 0x0 ;
mov r7, 0x1 ;
swi 0x0 ;
;
segment readable writeable ;
;
crlf db 0ah ;
;
msg db 'Input some data: ' ;
msg_size = $-msg ;
;
inp_buf rb 100h ;
inp_buf_size rw 1 ;
;
msg2 db 'You entered: ' ;
msg2_size = $-msg2 ;
; Code: Select all
;======================================================;
; Tiny Server. ;
;------------------------------------------------------;
; By Craig Bamford (Dex) (11-03-2013) ;
;------------------------------------------------------;
format ELF executable ;
entry start ;
;======================================================;
; Is logging enabled ? ;
;------------------------------------------------------;
LOGGING = 1 ;
LOGGING2 = 0 ;
;======================================================;
; Def ;
;------------------------------------------------------;
SOCK_STREAM = 1 ;
AF_INET = 2 ;
SYS_socketcall = 102 ;
SYS_SOCKET = 1 ;
SYS_CONNECT = 3 ;
SYS_SEND = 9 ;
SYS_RECV = 10 ;
IPPROTO_TCP = 6 ; Transmission Control Protocol
;======================================================;
; Print macro ;
;------------------------------------------------------;
macro print _msg,_msglen ;
{ common ;
push (r0) ;
push (r1) ;
push (r2) ;
push (r7) ;
mov r7, 0x4 ;
mov r0, 0x1 ;
imm32 r1, _msg ;
imm32 r2, _msglen ;
swi 0x0 ;
;
pop (r7) ;
pop (r2) ;
pop (r1) ;
pop (r0) ;
} ;
;======================================================;
; old tcpsend macro ;
;------------------------------------------------------;
macro tcpsend _msg,_msglen ;
{ common ;
push (r0) ;
push (r1) ;
push (r2) ;
push (r3) ;
push (r7) ;
imm32 r0, clientfd ;
ldr r0, [r0] ;
imm32 r1, _msg ;
imm32 r2, _msglen ;
mov r3, 0 ;
imm32 r7, 0x121 ;
swi 0x0 ;
pop (r7) ;
pop (r3) ;
pop (r2) ;
pop (r1) ;
pop (r0) ;
} ;
;======================================================;
; old tcpsend macro ;
;------------------------------------------------------;
; r0 = size to send ;
;------------------------------------------------------;
macro tcpsend_reg _msg ;
{ common ;
push (r0) ;
push (r1) ;
push (r2) ;
push (r3) ;
push (r7) ;
mov r2, r0 ;
imm32 r0, clientfd ;
ldr r0, [r0] ;
imm32 r1, _msg ;
mov r3, 0 ;
imm32 r7, 0x121 ;
swi 0x0 ;
pop (r7) ;
pop (r3) ;
pop (r2) ;
pop (r1) ;
pop (r0) ;
} ;
;======================================================;
; Print macro ;
;------------------------------------------------------;
; r0 = string size ;
;------------------------------------------------------;
macro print_reg _msg,_msglen ;
{ common ;
push (r0) ;
push (r1) ;
push (r2) ;
push (r7) ;
mov r2, r0 ;
mov r7, 0x4 ;
mov r0, 0x1 ;
imm32 r1, _msg ;
swi 0x0 ;
;
pop (r7) ;
pop (r2) ;
pop (r1) ;
pop (r0) ;
} ;
include 'FasmArm.inc' ;
segment readable writeable executable ;
;======================================================;
; Main start ;
;------------------------------------------------------;
start: ;
print msg,msg.msg_size ; print start message
;======================================================;
; Check commamd line for port number ;
;------------------------------------------------------;
ldmfd sp!, {r1} ;
imm32 r0, argc ;
str r1, [r0] ;
mov r0, 80 ; default port number
cmp r1, 1 ;
;======================================================;
; If no number, use default (port 80) ;
;------------------------------------------------------;
beq UseDeFault ;
;======================================================;
; Convet port number ;
;------------------------------------------------------;
ldmfd sp!,{r1} ;
ldmfd sp!,{r3} ;
mov r4, 10 ;
bl StrToInt ;
mov r0, r1 ;
imm32 r1, portnumber ;
str r0, [r1] ;
align 4 ;
UseDeFault: ;
;======================================================;
; Store Port ;
;------------------------------------------------------;
rev16 r0, r0 ; byte order
imm32 r1, sockaddr_in.sin_port ;
strh r0, [r1] ;
;======================================================;
; SOCKET(2) ;
;------------------------------------------------------;
; Creates an endpoint for communication and returns ;
; a descriptor. ;
;------------------------------------------------------; ;
imm32 r7, 0x119 ; socket sys number
mov r0, AF_INET ;
mov r1, SOCK_STREAM ;
mov r2, 0 ;
swi 0x0 ;
teq r0, 0 ;
bpl bind ; branch if not neg
;
mvn r0, r0 ;
add r0, 1 ; change neg to pos number
imm32 r1, 4096 ; We need to check because some times address is neg
cmp r0, r1 ;
bl Convert2Hex ; Get error code
print print_hex_string,print_hex_string_size ;
print msgE1,msgE1.msgE1_size ; error message 1
b fin ; branch exit
align 4 ;
;======================================================;
; BIND(2) ;
;------------------------------------------------------;
; When a socket is created with socket(2), ;
; it exists in a name space (address family) but has ;
; no address assigned to it. bind() assigns the ;
; address specified to by addr to the socket referred ;
; to by the file descriptor sockfd. addrlen specifies ;
; the size, in bytes, of the address structure pointed ;
; to by addr. Traditionally, this operation is called ;
; "assigning a name to a socket". ;
;------------------------------------------------------;
bind: ; Ok SOCKET(2)
imm32 r1, sockfd ;
str r0, [r1] ;
;
imm32 r0, sockfd ; bind(int sockfd, const struct sockaddr *addr,socklen_t addrlen);
ldr r0, [r0] ;
imm32 r1, sockaddr_in ;
imm32 r2, dest_size ;
imm32 r7, 0x11A ; BIND(2)
swi 0x0 ;
;
teq r0, 0 ; check for errors
bpl listen ; branch if not neg
;
mvn r0, r0 ;
add r0, 1 ; change neg to pos number
imm32 r1, 4096 ; We need to check because some times address is neg
cmp r0, r1 ;
bl Convert2Hex ; Get error code
print print_hex_string,print_hex_string_size ;
print msgE2,msgE2.msgE2_size ; error message 1
b fin ; branch exit
; Ok BIND(2)
;======================================================;
; LISTEN(2) ;
;------------------------------------------------------;
; Start to listen for incomming connections, ;
; allowing for a queue size of ;
; waiting connections of 20. ;
;------------------------------------------------------;
align 4 ;
listen: ;
imm32 r0, sockfd ; listen(int sockfd, int backlog);
ldr r0, [r0] ;
mov r1, 0 ; was 20 *****
imm32 r7, 0x11C ; LISTEN(2)
swi 0x0 ;
;
teq r0, 0 ; check for errors
bpl infinity ; branch if not neg
;
mvn r0, r0 ;
add r0, 1 ; change neg to pos number
imm32 r1, 4096 ; We need to check because some times address is neg
cmp r0, r1 ;
bl Convert2Hex ; Get error code
print print_hex_string,print_hex_string_size ;
print msgE3,msgE3.msgE3_size ; error message 1
b fin ; branch exit
align 4 ;
infinity: ; Ok LISTEN(2)
;======================================================;
; ACCEPT(2) ;
;------------------------------------------------------;
; The accept() system call is used with ;
; connection-based socket types (SOCK_STREAM, ;
; SOCK_SEQPACKET). It extracts the first connection ;
; request on the queue of pending connections for the ;
; listening socket, sockfd, creates a new connected ;
; socket, and returns a new file descriptor referring ;
; to that socket. The newly created socket is not in ;
; the listening state. The original socket sockfd is ;
; unaffected by this call. ;
;------------------------------------------------------;
accept_loop: ;
imm32 r0, sockfd ;
ldr r0, [r0] ;
imm32 r1, client_addr_ip ; client_addr
imm32 r2, sixteen ; addrlen
imm32 r7, 0x11D ; ACCEPT(2)
swi 0x0 ;
;
teq r0, 0 ; check for errors
bgt accept_ok ;
if LOGGING2 ;
mvn r0, r0 ;
add r0, 1 ; change neg to pos number
bl Convert2Hex ; Get error code
print print_hex_string,print_hex_string_size ;
print msgE4,msgE4.msgE4_size ; error message 1
end if ;
b accept_loop ; branch if neg or zero
align 4 ;
;======================================================;
; RECV(2) ;
;------------------------------------------------------;
; The recv(2) and recvmsg() calls are used to receive ;
; messages from a socket, and may be used to receive ;
; data on a socket whether or not it is ;
; connection-oriented. ;
; ;
; If src_addr is not NULL, and the underlying protocol ;
; provides the source address, this source address is ;
; filled in. When src_addr is NULL, nothing is filled ;
; in, in this case, addrlen is not used, and should ;
; also be NULL. The argument addrlen is a value-result ;
; argument, which the caller should initialize before ;
; the call to the size of the buffer associated with ;
; src_addr, and modified on return to indicate the ;
; actual size of the source address. The returned ;
; address is truncated if the buffer provided is too ;
; small, ;
; in this case, addrlen will return a value ;
; greater than was supplied to the call. ;
;------------------------------------------------------;
accept_ok: ;
imm32 r1, clientfd ;
str r0, [r1] ;
recv_loop: ;
imm32 r0, clientfd ;
ldr r0, [r0] ; client FD
imm32 r1, buffer ; buffer
imm32 r2, bufferlen ; buffer size
imm32 r3, 0 ; No options
imm32 r7, 0x123 ; RECV(2)
swi 0x0 ;
;
teq r0, 0 ; check for errors
blt recv_loop ;
imm32 r1, recv_size ; get size of message
str r0, [r1] ; store it
;======================================================;
; if logging2 then print buffer ;
;------------------------------------------------------;
if LOGGING2 ;
print buffer, bufferlen ;
;======================================================;
; Next line ;
;------------------------------------------------------;
print NextLine, NextLine.NextLine_size ;
;
end if ;
;======================================================;
; test for file ;
;------------------------------------------------------;
imm32 r0, buffer ;
ldr r2, [r0], 4 ;
imm32 r3, 'GET ' ;
cmp r2, r3 ;
bne close ;
eor r1, r1 ;
ldrb r2, [r0], 1 ;
cmp r2, '/' ;
bne close ;
filewantedloop: ;
ldrb r2, [r0], 1 ;
cmp r2, ' ' ;
beq ItsSpace ;
;
imm32 r3, bufferwantedfile ;
strb r2, [r3,r1] ;
add r1, 1 ;
b filewantedloop ;
ItsSpace: ;
imm32 r3, bufferwantedfile ;
cmp r1,0 ;
bne Notjustspace ;
imm32 r6,"inde" ;
str r6,[r3],4 ;
imm32 r6,"x.ht" ;
str r6,[r3],4 ;
imm32 r6,"m" ;
strb r6,[r3] ;
imm32 r3, bufferwantedfile ;
mov r1,9 ;
Notjustspace: ;
mov r4, 0 ;
strb r4, [r3,r1] ;
imm32 r4, offset ;
str r1, [r4] ;
;======================================================;
; Is logging enabled ? ;
;------------------------------------------------------;
; If logging has been enabled, itll use the function ;
; called ntop (at the bottom of this code) to convert ;
; the IP address of the person who connected to the ;
; server into a string form and print it. ;
;------------------------------------------------------;
if LOGGING ;
imm32 r2, client_addr_ip ;
ldr r0, [r2, 4] ; mov eax, dword [peeraddr+4]
imm32 r3, client_addr_ip_Converted ; mov edi, buffer+bufferlen-16
bl ntop ; call ntop
print_reg client_addr_ip_Converted ;
end if ;
;======================================================;
; get file info ;
;------------------------------------------------------;
fileisnamed: ;
mov r7, 106 ;
imm32 r0, bufferwantedfile ;
imm32 r1, stat ;
swi 0x0 ;
cmp r0, 0 ;
beq openfile ;
;
;======================================================;
; file not found ;
;------------------------------------------------------;
tcpsend h404,h404.len ;
;======================================================;
; if logging print 404 message ;
;------------------------------------------------------;
if LOGGING ;
print l404,l404.len ;
;======================================================;
; Print wanted file name ;
;------------------------------------------------------;
imm32 r2, offset ;
ldr r2, [r2] ;
mov r7, 0x4 ;
mov r0, 0x1 ;
imm32 r1, bufferwantedfile ;
;
swi 0x0 ;
;======================================================;
; Next line ;
;------------------------------------------------------;
print NextLine, NextLine.NextLine_size ;
end if ;
b close ;
;
;======================================================;
; Try to open file ;
;------------------------------------------------------;
openfile: ;
imm32 r4, stat ;
ldr r0, [r4,20] ;
imm32 r5, fdlen ;
str r0, [r5] ;
mov r7, 5 ;
imm32 r0, bufferwantedfile ;
mov r1 ,0 ;
mov r2 ,0 ;
swi 0x0 ;
teq r0, 0 ; check for errors
bgt fd_ok ;
;======================================================;
; Error when opeing file ;
;------------------------------------------------------;
mvn r0, r0 ;
add r0, 1 ; change neg to pos number
bl Convert2Hex ;
print print_hex_string, print_hex_string_size
print fderror,fderror_len ;
b close ;
;======================================================;
; file found and ready to send ;
;------------------------------------------------------;
fd_ok: ;
imm32 r1, fd ;
str r0, [r1] ;
;======================================================;
; if logging, print 200 OK message ;
;------------------------------------------------------;
if LOGGING ;
print l200,l200.len ;
;======================================================;
; Print wanted file name ;
;------------------------------------------------------;
imm32 r2, offset ;
ldr r2, [r2] ;
mov r7, 0x4 ;
mov r0, 0x1 ;
imm32 r1, bufferwantedfile ;
;
swi 0x0 ;
;======================================================;
; Next line ;
;------------------------------------------------------;
print NextLine, NextLine.NextLine_size ;
end if ;
;======================================================;
; set header (eg: file size) ;
;------------------------------------------------------;
imm32 r0, fdlen ;
ldr r0, [r0] ;
imm32 r10, c_l ;
bl IntToStr ;
mov r1, 13 ;
strb r1, [r10],1 ;
mov r1, 10 ;
strb r1, [r10],1 ;
mov r1, 13 ;
strb r1, [r10],1 ;
mov r1, 10 ;
strb r1, [r10],1 ;
;
imm32 r1, c_l ;
sub r10, r1 ;
imm32 r0, h200.len ;
add r0, r10 ;
; print NextLine, NextLine.NextLine_size ;
; print_reg h200 ;, h200.len ; temp
; print NextLine, NextLine.NextLine_size ;
;======================================================;
; send header ;
;------------------------------------------------------;
tcpsend_reg h200 ;
;======================================================;
; Send file ;
;------------------------------------------------------;
mov r7, 187 ;
imm32 r0, clientfd ;
ldr r0, [r0] ;
imm32 r1, fd ;
ldr r1, [r1] ;
mov r2, 0 ;
imm32 r3, fdlen ;
ldr r3, [r3] ;
mov r4, 0 ;
swi 0x0 ;
;
align 4 ;
;======================================================;
; Close socket and wait ;
;------------------------------------------------------;
close: ;
mov r7, 6 ;
imm32 r0, fd ;
ldr r0, [r0] ;
swi 0x0 ;
mov r7, 6 ; CLOSE(2)
imm32 r0, clientfd ;
ldr r0, [r0] ;
swi 0x0 ;
imm32 r0,buffer ;
eor r1,r1 ;
imm32 r2,bufferlen ;
mov r2,r2, lsr 2 ;
align 4 ;
ClearBuffLoop: ;
str r1,[r0],4 ;
subs r2,r2,1 ;
bne ClearBuffLoop ;
b infinity ;
align 4 ;
;======================================================;
; Close server and exit ;
;------------------------------------------------------;
fin: ; exit
print msgclose,msgclose.msgclose_size ;
mov r0, 0x0 ;
mov r7, 0x1 ;
swi 0x0 ;
;
;======================================================;
; Converts to hex ;
; r0 = number ( prints 8 hex digits) ;
;------------------------------------------------------;
align 4 ;
Convert2Hex: ;
stmfd sp!, {r0-r12, lr} ;
imm32 r2,hex_digits ;
imm32 r3,print_hex_string ;
mov r4,28 ;
align 4 ;
print_hex_loop: ;
mov r1,r0,lsr r4 ; Get digit n
and r1,r1,0x0f ; mask off lower nibble
ldrb r1,[r2,r1] ; r0 now contains a hex number,
; look it up in table
strb r1,[r3],1 ;
subs r4,r4,4 ;
bpl print_hex_loop ;
;
ldmfd sp!, {r0-r12, pc} ; pop & return
align 4 ;
;======================================================;
; String to int ;
; input: r3 = buffer, r4 = base (eg 10) ;
; output: r1 = number ;
;------------------------------------------------------;
StrToInt: ;
stmfd sp!, {r2-r3, lr} ; save regs
eor r1, r1 ; zero r1 and r2
eor r2, r2 ;
.loop: ;
ldrb r2, [r3], 1 ; load a byte
cmp r2, 0 ; check for 0
beq .end ; if so exit
mul r1, r1, r4 ; multiply with basic
sub r2, r2, '0' ; sub ASCII value
cmp r2, 9 ; cmpare it to 9
ble .ok ; if its let or = jump to OK
sub r2, 7 ; if not sub 7 (eg: it A-F)
.ok: ;
add r1, r1, r2 ; save it
b .loop ; do another loop .loop
.end: ;
ldmfd sp!, {r2-r3, pc} ; restore regs and return.
align 4 ;
;======================================================;
; write number to buffer ;
;------------------------------------------------------;
; r10 = buffer address ;
;------------------------------------------------------;
number2buffer: ;
stmfd sp!, {lr} ;
strb r0, [r10],1 ;
ldmfd sp!, {pc} ;
align 4 ;
;======================================================;
; Int To String ;
;------------------------------------------------------;
; input: r0 = number ;
; r10 = buffer ;
; ;
; output: r10 = buffer + offset ;
;------------------------------------------------------; ;
IntToStr: ;
stmfd sp!, {r0-r9, lr} ; save regs
imm32 r8,prt_digits ;
;
imm32 r6,0 ; current character to print
mov r5,9 ; digits - 1 (max pos value 2147483647) (31 bits)
imm32 r9,0 ;
;
;
cmp r0,0 ;
beq zeronum ;
and r7,r0,$80000000 ; test bit 31 of r0
cmp r7,0 ;
beq posnum ; go to posnum if result equals zero (positive)
;
imm32 r2,$FFFFFFFF ;
eor r0,r0,r2 ; invert all bits of r0 (convert to positive)
mov r2,r0 ; backup r0 value
add r2,r2,1 ; add 1 to (now positive) value
imm32 r0,"-" ;
bl number2buffer ; print negative (-) sign
mov r0,r2 ; restore r0 value
;
posnum: ;
imm32 r4,1000000000 ;
subs r1,r0,r4 ; subtract r4 (power of 10 result) from starting value, store in r1
b Printloop ;
;
align 4 ;
mainloop: ;
imm32 r6,0 ; clear r6
mov r2,r5 ; back up value of r5
b pow10 ; lookup power of 10 from r5
p10ret: ;
mov r5,r2 ; restore r5 value
subs r1,r1,r4 ; subtract r4 (power of 10 result) from starting value, store in r1
;
Printloop: ;
;print "At Printloop" ;
;sync ;
bpl notmatched ; go to notmatched if result (r1) is not negative
add r1,r1,r4 ; add power of 10 value to r1
cmp r6,0 ;
blt nodigit ; if r6 is negative, reset to 0
b numfix ;
notmatched: ;
;-print "at notmatched" ;
add r6,r6,1 ; increment current character by 1
subs r1,r1,r4 ; subtract pow10 value from r1
b Printloop ;
numfix: ;
mov r3,r6 ; update r3 from r6
ldrb r3,[r8,r3] ; r3 now contains a number (ascii)
mov r0,r3 ; move ascii character to r0
cmp r9,0 ; if r9 does not equal zero,
bne allowprt ; go to allowprt (allow printing)
cmp r0,'0' ; if r0 equals ascii zero,
beq noprint ; go to noprint (disable printing leading zeros)
imm32 r9,1 ; set r9 to 1 (allow printing zeros)
;
allowprt: ;
bl number2buffer ; print character to buffer
noprint: ;
subs r5,r5,1 ; decrement r5 (power of 10 mult) by 1
mov r0,r1 ; replace r0 with r1
bpl mainloop ; go to mainloop if r5 is not negative
;sync ;
;-end ;
finish: ;
;print "Done printing" ;
;sync ;
ldmfd sp!, {r0-r9, pc} ; restore regs and return.
zeronum: ;
imm32 r0,"0" ;
bl number2buffer ; print negative (-) sign
b finish ;
;
;
nodigit: ;
imm32 r6,0 ;
b numfix ;
;
pow10: ; calculate 10^x (r5 value) ;
;r4 will contain result, r5 specifies power of 10 to calc
imm32 r4,10 ;
cmp r5,1 ;
bgt p10mul ;
blt p10p1 ;
imm32 r4,10 ;
b p10ret ;
p10p1: ;
imm32 r4,1 ;
b p10ret ;
p10mul: ;
sub r5,r5,1 ;
p10loop: ;
imm32 r7,10 ;
mul r4,r4,r7 ;
sub r5,r5,1 ;
cmp r5,0 ;
bgt p10loop ;
b p10ret ;
align 4 ;
;======================================================;
; ntop ;
; r0 = network-order address, r3 = buffer ;
; returns: r10 = buffer + offset ;
;------------------------------------------------------;
ntop: ;
; eax = network-order address, edi = buffer ;
; returns: ecx = length ;
stmfd sp!, {r1-r12, lr} ; save regs push ebx edx ebp
mov r10,r3 ;
mov r6, r3 ;
mov r1, r0 ;
imm32 r2, 0x000000FF ;
and r0, r2 ;
mov r4, 2 ;
;num4 ;
bl IntToStr ;
imm32 r2, '.' ;
strb r2, [r10],1 ;
;num3 ;
mov r0, r1 ;
imm32 r2, 0x0000FF00 ;
and r0, r2 ;
mov r0, r0, lsr 8 ;
mov r4, 2 ;
bl IntToStr ;
imm32 r2, '.' ;
strb r2, [r10],1 ;
;num2 ;
mov r0, r1 ;
imm32 r2, 0x00FF0000 ;
and r0, r2 ;
mov r0, r0, lsr 16 ;
mov r4, 2 ;
bl IntToStr ;
imm32 r2, '.' ;
strb r2, [r10],1 ;
; num1 ;
mov r0, r1 ;
mov r0, r0, lsr 24 ;
mov r4, 2 ;
bl IntToStr ;
;
sub r10, r6 ;
mov r0, r10 ;
ldmfd sp!, {r1-r12, pc} ; restore regs and return.ret
; ---------------------------------------------------- ;
; OutChar ( print 1 character to stdout ) ;
; ---------------------------------------------------- ;
; put char = r0 ;
; ---------------------------------------------------- ;
OutChar: ;
stmfd sp!, {r0-r2, r7, lr} ; Store registers
mov r1, sp ; r1 address
mov r0, 1 ; r0 stdout
mov r2, r0 ; r2 length
mov r7, 4 ;
swi 0 ;
ldmfd sp!, {r0-r2, r7, pc} ; Restore registers and return
;======================================================;
; data ;
;------------------------------------------------------;
segment readable writeable ;
if LOGGING ;
align 4 ;
l404 db ' - 404 NF - ' ;
.len = $ - l404 ;
align 4 ;
l200 db ' - 200 OK - ' ;
.len = $ - l200 ;
end if ;
align 4 ;
ipnumber dw 0xfefefefe ;
align 4 ;
prt_digits: ;
db "0123456789" ;
align 4 ;
hex_digits: ;
db "0123456789ABCDEF" ;
align 4 ;
print_hex_string: ;
db "12345678", 0xa ; storage for 8 digit hex string,
print_hex_string_size = $-print_hex_string ;
align 4 ;
client_addr_ip: rb 16 ;
sixteen dw 16 ;
align 4 ;
sockaddr_in: ;
.sin_family dh AF_INET ;
.sin_port dh 0 ;
.sin_addr dw 0 ;
.sin_zero rb 8 ;
dest_size = $-sockaddr_in ;
align 4 ;
client_addr dw sockaddr_in ;
addrlen dw dest_size ;
align 4 ;
argc dw 0 ;
portnumber dw 0 ;
align 4 ;
recv_size dw 0 ;
sockfd dw 0 ;
fd dw 0 ;
fdlen dw 0 ;
clientfd dw 0 ;
offset dw 0 ;
align 4 ;
cArray dw 0 ;
dw 0 ;
dw 0 ;
dw 0 ;
dw 0 ;
align 4 ;
;send 'array'. ;
sArray dw 0 ;
dw 0 ;
dw 0 ;
dw 0 ;
align 4 ;
indexfile db ".index.htm",0 ;
align 4 ;
NextLine db " " ,0xa ; Next line
.NextLine_size = $-NextLine ;
align 4 ;
msgreceved db " Server receved data" ,0xa ; Message
.msgreceved_size = $-msgreceved ;
align 4 ;
msg db " Server started..." ,0xa ; Message
.msg_size = $-msg ;
align 4 ;
msgclose db " Server closed", 0xa ; Close
.msgclose_size = $-msgclose ;
align 4 ;
msgsent db " Sent file", 0xa ; Send
.msgsent_size = $-msgsent ;
;
msgE1 db " socket() failed" ,0xa ; Err1
.msgE1_size = $-msgE1 ;
align 4 ;
msgE2 db " bind() failed", 0xa ; Err2
.msgE2_size = $-msgE2 ;
align 4 ;
msgE3 db " listen() failed", 0xa ; Err3
.msgE3_size = $-msgE3 ;
align 4 ;
msgE4 db " accept() failed", 0xa ; Err4
.msgE4_size = $-msgE4 ;
align 4 ;
msgE5 db " send() failed", 0xa ; Err5
.msgE5_size = $-msgE5 ;
align 4 ;
fderror db " ERROR 1", 0xa ;
fderror_len = $- fderror ;
align 4 ;
fderror2 db " ERROR 2", 0xa ;
fderror_len2 = $- fderror2 ;
align 4 ;
h404 db 'HTTP/1.0 404 Not Found',13,10,13,10,'<HTML><HEAD><TITLE>404 Not Found</TITLE></HEAD><BODY><H1>File Not Found</H1></BODY></HTML>',10,13
.len = $ - h404 ;
align 4 ;
h200 db 'HTTP/1.0 200 OK',13,10 ;
db 'Server: DexServer',13,10 ;
db 'Content-Length: ' ;
.len = $ - h200 ;
align 4 ;
c_l db ' ';
align 4 ;
stat rb 88 ;
align 4 ;
buffer rb 512 ;
bufferlen = $ - buffer ;
align 4 ;
bufferwantedfile rb 512 ;
align 4 ;
client_addr_ip_Converted rb 16 ;
align 4 ;
Last edited by DexOS on Sat Mar 30, 2013 11:55 am, edited 1 time in total.
Batteries not included, Some assembly required.
Re: Asm tuts
You will need this for the above server
FasmArm.inc
As "jamesh" pointed out, that book will be a big help in understanding whats going on.
FasmArm.inc
Code: Select all
macro imm16 reg,immediate {
mov reg,(immediate) and $FF
orr reg,(immediate) and $FF00
}
macro imm16eq reg,immediate {
moveq reg,(immediate) and $FF
orreq reg,(immediate) and $FF00
}
macro imm16ne reg,immediate {
movne reg,(immediate) and $FF
orrne reg,(immediate) and $FF00
}
macro imm16lt reg,immediate {
movlt reg,(immediate) and $FF
orrlt reg,(immediate) and $FF00
}
macro imm16gt reg,immediate {
movgt reg,(immediate) and $FF
orrgt reg,(immediate) and $FF00
}
macro imm32 reg,immediate {
mov reg,(immediate) and $FF
orr reg,(immediate) and $FF00
orr reg,(immediate) and $FF0000
orr reg,(immediate) and $FF000000
}
macro imm32eq reg,immediate {
moveq reg,(immediate) and $FF
orreq reg,(immediate) and $FF00
orreq reg,(immediate) and $FF0000
orreq reg,(immediate) and $FF000000
}
macro imm32ne reg,immediate {
movne reg,(immediate) and $FF
orrne reg,(immediate) and $FF00
orrne reg,(immediate) and $FF0000
orrne reg,(immediate) and $FF000000
}
macro imm32lt reg,immediate {
movlt reg,(immediate) and $FF
orrlt reg,(immediate) and $FF00
orrlt reg,(immediate) and $FF0000
orrlt reg,(immediate) and $FF000000
}
macro imm32gt reg,immediate {
movgt reg,(immediate) and $FF
orrgt reg,(immediate) and $FF00
orrgt reg,(immediate) and $FF0000
orrgt reg,(immediate) and $FF000000
}
Batteries not included, Some assembly required.
Re: Asm tuts
its the os-specific bits that will be the problem though - you'd have to convert SWI calls to their linux equivalents, which for graphics/sound would be a nightmare; filehandling wouldn't be great either given the very different filesystems between adfs and ext4. floating point would be interesting as essentially its not there on riscos. riscos is 26-bit armv3 or earlier not 64-bit armv6 so you're limiting register usage and missing a lot of useful instructions and addresses would be different. with riscos you generally run assembler from the basic interpreter rather than building it with gcc/binutils so where would you learn that?jamesh wrote:Why not? The underlying assembly language is exactly the same between RISCOS and Linux (and anything else on the Raspi) so all the principles apply. And it's so easy to move to and from OS's on the Raspi..sej7278 wrote:that's for riscos so won't do you much good.malakai wrote:I would love to see some. I still have this book on my things to get list http://www.amazon.com/Raspberry-Pi-Asse ... 148112790X I think maybe some easy games or anything could be helpful.
i'd love to see a single program (even hello world!) that could run on linux and riscos without modification.
a linux assembler book would be better for learning linux assembler, even an android assembler book would be better than riscos.
don't get me wrong, i learnt arm assembler on riscos, but today its just a pointless place to start.
-
antiloquax
- Posts: 406
- Joined: Sun Nov 20, 2011 11:37 am
- Contact: Website
Re: Asm tuts
@dexOS
Many thanks for your code examples.
I've started a little blog to record how my learning of asm is going.
asm blog
Many thanks for your code examples.
I've started a little blog to record how my learning of asm is going.
asm blog
Re: Asm tuts
Your welcome and blogs like your's are a good way to keep a record of your progress and helps others.antiloquax wrote:@dexOS
Many thanks for your code examples.
I've started a little blog to record how my learning of asm is going.
asm blog
Batteries not included, Some assembly required.
Re: Asm tuts
Rubbish.sej7278 wrote: a linux assembler book would be better for learning linux assembler, even an android assembler book would be better than riscos.
don't get me wrong, i learnt arm assembler on riscos, but today its just a pointless place to start.
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Contrary to popular belief, humorous signatures are allowed.
I've been saying "Mucho" to my Spanish friend a lot more lately. It means a lot to him.
Contrary to popular belief, humorous signatures are allowed.
I've been saying "Mucho" to my Spanish friend a lot more lately. It means a lot to him.
-
antiloquax
- Posts: 406
- Joined: Sun Nov 20, 2011 11:37 am
- Contact: Website
Re: Asm tuts
Regarding the RISCos book. I am keen to do "pure" asm in Linux using the GNU assembler, but I am finding that book helpful.
For instance, I just used it to help me refactor some code from the book "Programming from the Ground Up" into ARM assembly code.
It's the program to find the maximum from a series of numbers.
Bruce Smith's book made it a lot easier for me to figure out how to step through a series of numbers.
And thank you to dexOS for your words of encouragement. Is it okay with you if I post up versions of your examples (once I've got to grips with them and got them working with GAS)?
mark
For instance, I just used it to help me refactor some code from the book "Programming from the Ground Up" into ARM assembly code.
It's the program to find the maximum from a series of numbers.
Bruce Smith's book made it a lot easier for me to figure out how to step through a series of numbers.
And thank you to dexOS for your words of encouragement. Is it okay with you if I post up versions of your examples (once I've got to grips with them and got them working with GAS)?
mark
Re: Asm tuts
we seem to disagree on pretty much everything, so i'm walking away from this one.jamesh wrote:Rubbish.sej7278 wrote: a linux assembler book would be better for learning linux assembler, even an android assembler book would be better than riscos.
don't get me wrong, i learnt arm assembler on riscos, but today its just a pointless place to start.
Re: Asm tuts
Sure that will be fineantiloquax wrote:And thank you to dexOS for your words of encouragement. Is it okay with you if I post up versions of your examples (once I've got to grips with them and got them working with GAS)?
mark
.I have just done my first RiscOS hello world using FasmArm, which was not simple as it does not output the right file format, but luckly i found a example of the smallest header needed on the net and converted it to fasmarm.
In case it helps others:
Code: Select all
format binary as ''
OS_Write0 = 0x02 ; Define the two system calls
OS_Exit = 0x11 ; used in this code
;
org 0x8000 ; org (may not be needed ?
use32 ; use 32bit
mov r0,r0 ; Decompression code call
mov r0,r0 ; Self-relocation code call
mov r0,r0 ; Zero initialisation code call
bl start ; Program entry call
swi OS_Exit ; Fall-out trap to force exit
dw 0x40 ; Read-only area size (header)
dw 0x20 ; Read-write area size (code)
dw 0 ; Debug area size
dw 0 ; Zero initialisation size
dw 0 ; Debug type
dw 0x8000 ; Current base of absolute
dw 0 ; Workspace required
dw 32 ; Flag software as 32 bit PCR okay
dw 0 ; Data base address when linked
dw 0 ; Reserved header (should be zero)
dw 0 ; Reserved header (should be zero
message:
db "Hello World! :-)",0xa ,0
align 4
start:
adr r0,message ; Pointer to message
swi OS_Write0 ; OS call writes until null byte
mov r0,0 ; Define return code
swi OS_Exit ; And exit.
Batteries not included, Some assembly required.
-
antiloquax
- Posts: 406
- Joined: Sun Nov 20, 2011 11:37 am
- Contact: Website
Re: Asm tuts
Hi dex,
What I want to do is, write numbers to stdout.
I think I can manage to convert the binary to a sequence of ascii character codes. I will prob push them onto a stack so I can pop them off in the right order.
The problem is that I can't work out how to print.
Can I load a number into r1 and have it print the character to the screen?
What I want to do is, write numbers to stdout.
I think I can manage to convert the binary to a sequence of ascii character codes. I will prob push them onto a stack so I can pop them off in the right order.
The problem is that I can't work out how to print.
Can I load a number into r1 and have it print the character to the screen?
Re: Asm tuts
Hi antiloquax, yes you can, but you will need to use stack.
Here's a print char function example
Hope it helps and If you need more help just ask.
Here's a print char function example
Code: Select all
; Char function
format ELF executable
entry start
include 'ArmMacro.inc'
segment readable writeable executable
start: ;
mov r0, 5 ; number 5
add r0, 0x30 ; make ascii
bl PrintChar ; call function
;
mov r0, 0ah ; next line
bl PrintChar ; call function
mov r0, 0x0 ; exit
mov r7, 0x1 ;
swi 0x0 ;
PrintChar: ;
stmfd sp!, {r0-r2, r7, lr}; save regs
mov r1, sp ; use sp to point to r0
mov r0, 1 ; stdout
mov r2, r0 ; just one char
mov r7, 4 ; write
swi 0 ;
ldmfd sp!, {r0-r2, r7, pc}; restore regs
;
segment readable writeable ;Batteries not included, Some assembly required.
-
antiloquax
- Posts: 406
- Joined: Sun Nov 20, 2011 11:37 am
- Contact: Website
Re: Asm tuts
Thanks DEXos, I will have a go with that tomorrow.
-
coyotebush
- Posts: 12
- Joined: Fri Mar 01, 2013 7:39 pm
Re: Asm tuts
Being an old-time assembly programmer, I'll have to say nobody writes pure assembly any more. Instead, most code is written in "C" (or C++), and the critical pieces of assembly are written as "inlined assembly".
In applications, the main use of assembly is to access atomic operands (eg. test and set) or to optimize compression or encryption algorithms. Occasionally, an embedded application will implement a very low level I/O routine or a timing loop, but those don't work well from within a Linux environment where there are memory management issues, interrupts and DMA transfers.
At any rate, I'd vote for a tutorial on using ARM inline assembly rather than writing a complete assembly program.
In applications, the main use of assembly is to access atomic operands (eg. test and set) or to optimize compression or encryption algorithms. Occasionally, an embedded application will implement a very low level I/O routine or a timing loop, but those don't work well from within a Linux environment where there are memory management issues, interrupts and DMA transfers.
At any rate, I'd vote for a tutorial on using ARM inline assembly rather than writing a complete assembly program.
-
antiloquax
- Posts: 406
- Joined: Sun Nov 20, 2011 11:37 am
- Contact: Website
Re: Asm tuts
Thanks DEXos, that worked great (after a little tweaking because I am using gas).
I now have a program that can convert a .long to ascii chars!
thanks again.
I now have a program that can convert a .long to ascii chars!
thanks again.
Re: Asm tuts
@antiloquax, Glad you got it working.
@coyotebush, You are right to a point, but there's are some weird coders like myself, that like assembly as a programming language, we do not code in it because we think it will make our code faster, just by coding in it (unless we optimize it), but because we like the language and we want to learn how things work at a low level.
I mean in real terms what do people learn by, using per built lib and adding 10 lines of code ?.
You yourself know whats going on at a low level, because you have done assembly coding in the past.
But new coders may not know low level stuff and need to learn it, if they want to fully understand HLL.
@coyotebush, You are right to a point, but there's are some weird coders like myself, that like assembly as a programming language, we do not code in it because we think it will make our code faster, just by coding in it (unless we optimize it), but because we like the language and we want to learn how things work at a low level.
I mean in real terms what do people learn by, using per built lib and adding 10 lines of code ?.
You yourself know whats going on at a low level, because you have done assembly coding in the past.
But new coders may not know low level stuff and need to learn it, if they want to fully understand HLL.
Batteries not included, Some assembly required.
-
antiloquax
- Posts: 406
- Joined: Sun Nov 20, 2011 11:37 am
- Contact: Website
Re: Asm tuts
Yes - to echo what DEXos said, I am interested in doing some asm because I want to learn more about what is happening on the low level.
The rest of the time I am happy to use HLLs like Python, Scheme and Scala.
@ DEXos - can I just ask why we need to use the stack to set up those writes to stdout? Does the write process itself clobber the registers?
Here's what I have come up with for converting an integer into a series of ascii codes and printing them out (in the right order
).
I hope you don't mind me posting it here. If I have done something silly, I'd love some pointers.
The rest of the time I am happy to use HLLs like Python, Scheme and Scala.
@ DEXos - can I just ask why we need to use the stack to set up those writes to stdout? Does the write process itself clobber the registers?
Here's what I have come up with for converting an integer into a series of ascii codes and printing them out (in the right order
).I hope you don't mind me posting it here. If I have done something silly, I'd love some pointers.
Code: Select all
@ number.s
@ test program to print a number.
.section .data
num:
.long 123456791
.globl _start
_start:
mov r0, $0xA @ load r0 with ascii newline code
stmfd sp!, {r0} @ push onto stack
adr r3, num @ load r3 with address of num
ldr r4, [r3] @ load r4 with the number
mov r5, $0 @ set counter to zero
loop:
cmp r4, $9 @ if r4 <= 9 ...
ble stackPush @ branch ...
sub r4, r4, $10 @ subtract 10 from r4
add r5, r5, $1 @ add one to counter
bal loop @ back to top of loop
stackPush:
add r0, r4, $0x30 @ load r0 with r4 + 48
stmfd sp!, {r0} @ push onto stack
cmp r5, $0 @ if the counter is zero ...
beq stackPull @ prepare to print
mov r4, r5 @ load count into r4
mov r5, $0 @ reset counter
bal loop @ back to top of loop
stackPull:
ldmfd sp!, {r0} @ pull from stack
bl PrintChar @ call print function
cmp r0, $0xA @ was it a newline?
beq exit @ if so exit
bal stackPull @ otherwise get next char
exit:
mov r0, $0 @ exit
mov r7, $1
svc $0
PrintChar:
stmfd sp!, {r0-r5, r7, lr} @ push registers to stack
mov r1, sp @ use stack pointer
mov r0, $1 @ stdout
mov r2, r0 @ one char
mov r7, $4 @ write
swi $0
ldmfd sp!, {r0-r5, r7, pc} @ restore registers
.end
Re: Asm tuts
Hi antiloquax, we do not need to use stack, its just convenient, as that function needs a pointer to the char or chars, not the chars them self, so you could for example do this
And use a var instead, but its simpler to use stack.
Code: Select all
; Char2 function
format ELF executable
entry start
include 'ArmMacro.inc'
segment readable writeable executable
start: load_reg r1, TheChar ; load the address of TheChar into r1
mov r0, "5" ; load the number char into r0
Strb r0,[r1] ; load byte whats in r0 into address thats in r1
;note: r1 points to address TheChar still
mov r0, 1 ;
mov r2, r0 ;
mov r7, 4 ;
swi 0 ;
;
mov r0, 0x0 ; exit
mov r7, 0x1 ;
swi 0x0 ;
;
segment readable writeable ;
;
TheChar db 0 ; char var
And use a var instead, but its simpler to use stack.
Batteries not included, Some assembly required.
-
antiloquax
- Posts: 406
- Joined: Sun Nov 20, 2011 11:37 am
- Contact: Website
Re: Asm tuts
Hi Dex,
Thanks for your help again. I've been playing around with my print a number code and I think I have a better understanding of the stack.
Blog post here.
I have been having a go at capturing input, but failing to get the code to move on once the user hits enter.
I'll look back over some of your examples as I think I have missed something obvious.
If you know of any helpful sites to look at I'd appreciate it, then I won't need to keep asking dumb questions here.
Thanks for your help again. I've been playing around with my print a number code and I think I have a better understanding of the stack.
Blog post here.
I have been having a go at capturing input, but failing to get the code to move on once the user hits enter.
I'll look back over some of your examples as I think I have missed something obvious.
If you know of any helpful sites to look at I'd appreciate it, then I won't need to keep asking dumb questions here.
Re: Asm tuts
No ? is dumb, you want to learn so its natural you need answers.
First thing to note about learning, that others look down on.
Is to learn anything you need to COPY, its the only way to learn.
So find the code that's close to what you want and copy it, in this case its most likely to be for x86 linux, so convert it to arm.
If you study the code you will see that to port x86 linux assembly to linux arm assembly you will need to change the x86 reg to the arm reg, plus change the "int 80h" to "swi 0x0"
Once you got the code working, start moding and learning how it works.
Remember one big mistake many beginners make is that they thing they must know everything about how every bit of the code works.
That's how i was and i never learn't anything, it was not until i started to just make the code work anyway i could, including copying others work, that i started to learn.
You only learn by coding, not reading about coding, but by coding.
So first thing you need to do it watch these videos, there not about assembly, but about copying and learning
http://www.everythingisaremix.info/watch-the-series/
And this may help you with assembly:
http://asm.sourceforge.net/resources.html
First thing to note about learning, that others look down on.
Is to learn anything you need to COPY, its the only way to learn.
So find the code that's close to what you want and copy it, in this case its most likely to be for x86 linux, so convert it to arm.
If you study the code you will see that to port x86 linux assembly to linux arm assembly you will need to change the x86 reg to the arm reg, plus change the "int 80h" to "swi 0x0"
Once you got the code working, start moding and learning how it works.
Remember one big mistake many beginners make is that they thing they must know everything about how every bit of the code works.
That's how i was and i never learn't anything, it was not until i started to just make the code work anyway i could, including copying others work, that i started to learn.
You only learn by coding, not reading about coding, but by coding.
So first thing you need to do it watch these videos, there not about assembly, but about copying and learning
http://www.everythingisaremix.info/watch-the-series/
And this may help you with assembly:
http://asm.sourceforge.net/resources.html
Batteries not included, Some assembly required.