Program Segment Prefix: Difference between revisions

Revision as of 04:23, 28 September 2019

Revision as of 04:08, 19 November 2019

Line 119:

mov bl, [80h]

cmp bl, 7Eh

ja exit ; preventing overflow

mov byte [bx + 81h], '$'

Line 128:

exit:

mov ax, 4C00h ; subfunction 4C (see below)

int 21h

</source>

Line 135:

<source lang="nasm">

; save...

push ds

xor ax, ax

push ax

; move to the default data group (@data)

mov ax,@data

mov ds,ax, @data

mov dxds,mess1 ax

; print message in mess1 (21h subfunction 9)

mov dx,mess1

mov ahdx,9h mess1

mov axah,@data 9

int 21h

retf

</source>

If the executable was a .COM file, this procedure was unnecessary and the program could be terminated merely with a direct INT 20h instruction or else calling INT 21h Function 0,. howeverHowever, the programmer still had to ensure that the CS register contained the segment address of the PSP at program termination. Thus,

<source lang="nasm">

jmp start

Line 166:

int 20h

</source>