Before a program receives control from MS-DOS, its program segment prefix is set up to contain certain vital information, such as:Obtains the segment (paragraph) address of the program segment prefix (PSP) for the currently executing program.
Call with:
AH = 62H
Returns:
BX = segment address of program segment prefix
Notes:
Before a program receives control from MS-DOS, its program segment prefix is set up to contain certain vital information, such as:
the segment address of the program's environment block
the command line originally entered by the user
the original contents of the terminate, Ctrl-C, and critical-error handler vectors
the top address of available RAM
The segment address of the PSP is normally passed to the program in registers DS and ES when it initially receives control from MS-DOS. This function allows a program to conveniently recover the PSP address at any point during its execution, without having to save it at program entry.
Example:
Get the segment base of the program segment prefix, then copy the command tail from the PSP into the local buffer named buff.
ctail equ 080H ; PSP offset, command tail
buff db 80 dup (?) ; copy of command tail
.
.
.
; get PSP address
mov ah,62H ; function number
int 21h ; transfer to MS-DOS
; copy command tail
mov ds,bx ; PSP segment to DS
mov si,offset ctail ; offset of command tail
mov di,seg buff ; local buffer address
mov es,di
mov di,offset buff
mov cl,[si] ; length of command tail
inc cl ; include count byte
xor ch,ch
cld
rep movsb ; copy to local buffer
.
.
.