Once you get booted into the wimp, click on the apps icon on the icon bar, you will find two editors there, !StrongED is a programmers editor, and !Edit is a standard editor (with some programming features). As for an assembler, you can use the BBC BASIC V assembler that is built into the OS, or you can download !ASM, or !extASM, these are both quite good assemblers, you will need to track down a linker if you do any multi module stuff, as you likely will.
We are lucky in that on the Raspberry Pi coding for bare metal is made so simple, as we can do the coding and assembling in RISC OS with out any cavets at all, save the new kernel to an SD CARD in a USB Card reader, power down, and switch cards for testing. It is difficult to get a simpler setup for bare metal programming (unless you have two RPi, then do the coding in RISC OS on one, save it to an SD card in a USB card reader, and pop the card in the other, and go).
I hope this helps.
The Raspberry Pi is an ARM based computer, that runs many different and varied Operating Systems, including Linux, RISC OS, BSD, as well as many more.