Category : Files from Magazines
Archive   : VOL8N17.ZIP
Filename : FLAGS.ASM

;********** FLAGS.ASM

;Syntax: CALL ClearAll 'clear all flags
; CALL ClearFlag(FlagNum%) 'clear flag number FlagNum%
; CALL SetFlag(FlagNum%) 'set flag number FlagNum%
; Flag = GetFlag%(FlagNum%) 'retrieve flag number FlagNum%

.Model Medium, Basic
.Code

Public ClearAll, ClearFlag, SetFlag, GetFlag

ClearAll Proc Far
Push CS ;copy the code segment into ES
Pop ES
Mov DI,Offset CS:Flags ;point DI to the flag words
Mov CX,4 ;clear 4 words
Xor AX,AX ;set AX = 0
Rep Stosw ;store 4 zero words
Ret ;return to BASIC
ClearAll Endp

ClearFlag Proc Far
Push BP ;set BP to incoming variable
Mov BP,SP
Mov SI,[BP+06] ;get the address for FlagNum%
Call DoFlag ;let DoFlag handle the common code
Jc ExitClear ;if carry is set, FlagNum% was illegal
Not AL ;reverse to clear the flag
And CS:Flags [BX],AL ;set the appropriate Flag bit

ExitClear:
Pop BP ;restore BP
Ret 2 ;return to BASIC
ClearFlag Endp

SetFlag Proc Far
Push BP ;set BP to incoming variable
Mov BP,SP
Mov SI,[BP+06] ;get the address for FlagNum%
Call DoFlag ;let DoFlag do the work
Jc ExitSet ;if carry is set, FlagNum% was illegal
Or CS:Flags [BX],AL ;set the appropriate Flag bit

ExitSet:
Pop BP ;restore BP
Ret 2 ;return to BASIC
SetFlag Endp

GetFlag Proc Far
Push BP ;set BP to incoming variable
Mov BP,SP
Mov SI,[BP+06] ;get the address for FlagNum%
Call DoFlag ;let DoFlag worry about it
Test CS:Flags [BX],AL ;see if the bit is set
Mov AX,0 ;assume it isn't
Jz ExitGet ;we were right
Dec AX ;no, assign -1 for the function output

ExitGet:
Pop BP ;restore BP
Ret 2 ;return to BASIC
GetFlag Endp

DoFlag Proc Near ;common code for the other routines
Mov AX,[SI] ;put FlagNum% into AX
Dec AX ;adjust 1-64 to 0-63
Cmp AX,63 ;see if it's too high
Ja DoError ;it is, get out

Push AX ;save AX for later
Mov CX,8 ;divide by eight
Xor DX,DX ;clear DX for the divide
Div CX ;now AX points to the correct flag word
Mov BX,AX ;transfer to BX for an index

Mov AL,1 ;make a bit
Pop CX ;retrieve the flag number
Rol AL,CL ;now the bit is in position
Clc ;clear the carry to show no error
Ret ;return to caller

DoError:
Stc ;set the carry to show an error
Ret ;return to caller
DoFlag Endp

Flags DB 8 Dup (0) ;this holds the flags

End