Compilation of various source - basmain

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 0:4 F2H.

; 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!







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