I wrote it because I can never remember what the `test` instruction does to the zero flag. Every time I use the instruction I have to look up the docs. Looking up docs is fine, but running code in a REPL helps me remember things better.

It's a shame that modern debuggers don't have mini-assemblers included like the original Apple II. Having a REPL would be real nice. For one I wouldn't have to type 90 (NOP) into memory windows to blank out code like non mortal ASSERTS.

I believe in most (if not all) architectures that have it, a test instruction is the same as comparing to zero; so testing zero sets the zero flag :)

TEST instruction in x86/x64 is the same as AND instruction (bitwise and), but result of the computation is discarded (only flags are retained).

Thanks; my assembly experience was with earlier processors, with a single argument for their test instruction (kind of like calling x86 test with two same arguments). I should have checked what the x86 test instruction does before replying.

I do a lot of program analysis work, and it's occasionally useful to see the pre- and post-machine states of arbitrary instructions. I have my own (more? less?) hacky version of this program that I use for that purpose; I know other people use GEF and similar GDB extensions for similar purposes.

For visually exploring the results of applying instructions. Similar to how you would use jshell.

Education

Learning assembly can be a pain, especially without something like gdb (with layout regs &layout asm). This is much simpler and doesn't require you to type like 4-5 extra commands(start gdb, put breakpoint, set layouts, step through the code),thus avoiding the pain that gdb can be for very-simple asm programs.

Shellcode testing I suppose.