这个都是过去DOS时代的汇编源码，虽然已经过去了，但是对于学习汇编还是有帮助的，汇编语言只是程序员一门基础语言，大多人掌握即可，不一定要深入研究.......
extrn $$main:far
cseg segment para public 'code'
; This program is used to set the PSP address for a compiled BASIC program.
; The PSP segment is saved at 0:4F2H.
; It can also be used to limit the maximum memory available to a compiled
; BASIC program. The option '/M:nnn' is used on the command line, where
; nnn is the number of K-bytes the program is limited to. If no, /M option
; is specified, no memory limitation takes place. For example, '/M:64' would
; limit the program to 64*1024 bytes. The range for nnn is 64 to 1024.
; This routine gets control before BASIC, does its handiwork, and then
; passes control to the BASIC program. It must be linked as follows:
; LINK BASMAIN+yourprog,yourprog,NUL.MAP,BASCOM
; If BASMAIN is unable to limit memory as requested, a message is displayed
; and the execution of the program is continued.
public basmain
basmain proc far
assume cs:cseg,ds:cseg,ss:nothing,es:nothing
push ds ; save ds
xor ax,ax
mov ds,ax ; ds=0
mov si,4f2h ; dos communications area
mov ax,es ; get psp seg
mov [si],ax ; save psp in dos comm area
pop ds ; restore ds
mov si,80h ; point to command line
mov ch,0
mov cl,[si] ; get length of command line
jcxz p025 ; it's zero
p010: inc si
mov al,[si] ; get char from command line
cmp al,'/' ; is it a slash?
jnz p020 ; no
mov ax,[si+1] ; get next 2 chars
cmp ax,':M' ; is it M: ?
jz p030 ; yes
cmp ax,':m' ; is it m: ?
jz p030 ; yes
p020: loop p010 ; check next char
p025: jmp p080 ; no /m: or /M: found
p030: ; found /m: or /M:
add si,3 ; point to first number
mov ax,0
mov bx,0
mov cx,10
p040: mov bl,[si] ; get character
cmp bl,'0' ; out of range?
jb p050 ; yes
cmp bl,'9' ; out of range?
ja p050 ; yes
sub bl,'0' ; convert to binary
mul cx ; multiply ax by 10
add ax,bx ; add new digit
inc si ; point to next char
jmp p040 ; continue
p050: ; got value in ax
cmp ax,64 ; less than 64K?
jb p060 ; yes - print msg
cmp ax,1024 ; greater than 1024K?
ja p060 ; yes - print msg
mov cl,6
sal ax,cl ; convert from KB to paragraphs (*64)
mov bx,es ; get psp
add bx,ax ; new top of memory
mov si,2 ; point to top of memory in psp
mov ax,[si] ; get current top of memory
cmp ax,bx ; is new setting larger?
jae p055 ; no
mov dx,offset msg2 ; yes - print msg
jmp p065
p055: mov [si],bx ; save new top of memory
jmp p080
p060: mov dx,offset msg1 ; print the message
p065: add dx,100h ; fudge for the psp
mov ah,9
int 21h
p080: jmp $$main ; jump to BASIC's start point
msg1 db 'Memory specification must be from 64 to 1024',7,10,13,'$'
msg2 db 'Unable to limit memory',7,10,13,'$'
basmain endp
cseg ends
end basmain ; must be a main program!