Day 13

The Program Segment Prefix

	One of the most unused things in DOS! COM programs (what we're doing), are loading into their
segment starting at 0100h. From 0000h to 00FFh is all the Program Segment Prefix or PSP. The PSP has
alot of data as you could see by the link below.

Layout
	There's a good listing of the layout of the PSP here.

An Older Way To Exit
	Back in the day, Interrupt Service 20h was exit program. However, once multisegment EXEs came
along, it became outdated. However, the machine code for INT 20h exists at offset 0 in our segment, so
to jump there and exit the program, we could just do:

	jmp 0   ; will exit program.

Usefull! However, if you ever plan on making an actual EXE, you'll need to stick with function 4Ch of
DOS Service 21h. Interrupt 20h will not work with EXEs because of some wierd expectation with the value
of CS, which you REALLY should not mess with in EXEs.

The Command Arguments
	Any command line parameters are stored starting at 0081h in the PSP. The byte at 0080h gives
the length in bytes of the parameters. So now you know why DOS will say "Command line too long" or
something like that if the parameters are over 256 chars long.

	Anyway, it's a quite simple matter to print out the command line parameters, here's a small
program that does it:

;;---CODE START---;;
call PrintPSP  ; call the PrintPSP routine

ExitProgram:
	jmp 0  ; INT 20h is at offset 0 in the segment

PrintPSP:
	mov cl,[0080h]   ; get the length of the command arguments
	mov bx,0081h     ; load the address of the start
	cmp bx,0         ; if 0
	je ExitProgram   ; Exit the program.
PSP:
	add bx,1         ; add 1 to the address (the parameters start at 0081h, but there's usually a space there).
	mov dl,[bx]      ; get the char into DL
	mov ah,02h       ; specify Function 2 (Print Char)
	int 21h          ; call DOS Services
	loop PSP         ; if CL (the length) isn't 0, loop back up to PSP.
	ret              ; return to the instruction after the call, which is "jmp 0", which exits the program.
;;---CODE END---;;

Fun, isn't it?

This Day In Review

	The PSP is a fun thing to play around with!

	Happy coding!,
		- Mike H


Intro - Day 14