INT 0x21 - 0x4b - DOS 2+ - "EXEC" - Load and/or Execute Program

reg value descr
AH 0x4b
AL type of load
DS:DX ASCIIz program name (must include extentsion)
ES:BX parameter block (see below)
Format of EXEC parameter block for AL=00h,01h,04h:
Offset Size Description
0x00 WORD segment of environment to copy for child process (copy caller's environment if 0000h)
0x02 DWORD pointer to command tail to be copied into child's PSP
0x06 DWORD pointer to first FCB to be copied into child's PSP
0x0A DWORD pointer to second FCB to be copied into child's PSP
0x0E DWORD (AL=01h) will hold subprogram's initial SS:SP on return
0x12 DWORD (AL=01h) will hold entry point (CS:IP) on return
Format of EXEC parameter block for AL=03h:
Offset Size Description
0x00 WORD segment at which to load overlay
0x02 WORD relocation factor to apply to overlay if in .EXE format

CL cleared if successfull

  • BX, DX are destroyed
  • if sub function 0x01 is called, process ID set to new program's PSP can be requested via Int 0x21, 0x62

CL is set on error

  • DOS 2.x destroys all registers, including SS:SP
  • for function 00h and 01h, the calling process must ensure that there is enough unallocated memory available; if necessary, by releasing memory with AH=49h or AH=4Ah
  • for function 01h, the value to be passed to the child program is put on top of the child's stack
  • for function 03h, DOS assumes that the overlay is being loaded into memory allocated by the caller
  • function 01h has been documented for DOS 5+, but was undocumented in prior versions
  • some versions (such as DR-DOS 6.0) check the parameters and parameter block and return an error if an invalid value (such as an offset of FFFFh) is found

BUG: DOS 2.00 assumes that DS points at the current program's PSP


  • back2root/ibm-pc-ms-dos/interrupts/int_21/int_21_4b.txt
  • Dernière modification : 2023/01/16 17:54
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