Oh man you gave me flashbacks. The TRS-80 Model II's OS, TRSDOS-II, had a built-in debugger that was little more than a monitor. You could step through instruct

Oh man you gave me flashbacks. The TRS-80 Model II's OS, TRSDOS-II, had a built-in debugger that was little more than a monitor. You could step through instructions, examine and write to memory, set breakpoints to absolute memory locations, and that was it. I remember hand-assembling tiny Z80 programs in that thing and jumping into them, just to test my understanding of how machine code programs worked and how the computer executed them, and being super thrilled when I could get an A to appear somewhere on the screen or something. The machine had a much more complete assembly language programming toolkit which I also used to write more sophisticated programs, employing this debugger to examine them. But I felt like I'd "cracked the code" of the computer when I plugged hex numbers into RAM and then ran them straight from there. Most CPU ISA documentation should give you the opcodes that correspond to instruction mnemonics. You may have to plug in your own operands (registers, etc.) into bit fields in the instruction encoding. If you're serious about hand-assembling to begin with this should be no problem.