Used by debug.com and other similar debuggers to load the executable.

AX=4B00h ("exec") and AX=4B01h ("load") are very similar, they only differ in the way they exit from the syscall. 4B00h simply sets up the initial AX, DS, ES, SS:SP and CS:IP for the child process. 4B01h instead pushes what would be AX onto the child stack, then saves SS:SP and CS:IP into the exec parameter block, and returns to the caller normally. Everything else, including child PSP and whatnot, is set up identically for both calls.

It seems emu2 implements 4B00h by fork+execing another instance of the emulator. For 4B01h, the loading would have to be done in the same process.

References:

• rbil: INT 21/AH=4Bh
• rbil: EXEC parameter block
• ldebug maintainer's explanation on how to use 4B01h