Assembly Source series of Basmain

This is the past DOS era of the compilation of source code, although has passed, but for the study of the assembly is still helpful, assembly language is just a basic programmer language, most people can grasp, not necessarily in-depth research.

EXTRN $ $main: Far
CSEG segment para Public ' code '
; This are used to set's 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 are limited to. If No,/M option
; is specified, no memory limitation takes place. For example, '/m:64 ' would
; Limit to 64*1024 bytes. The range for NNN is 1024.
; This is 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 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 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-top of the 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 is from 1024 ', 7,10,13, ' $ '
MSG2 DB ' Unable to limit memory ', 7,10,13, ' $ '
Basmain ENDP
Cseg ends
End Basmain; Must be a main program!