Category : C++ Source Code
Archive   : VCCRT2.ZIP
Filename : UNGETCH.ASM

page ,132
title ungetch.asm - push back a character from getch/getche
;***
;ungetch.asm - contains ungetch()
;
; Copyright (c) 1985-1992, Microsoft Corporation. All rights reserved.
;
;Purpose:
; The routine "ungetch()" pushes back a character to be returned
; by the next call to either "getch()" or "getche()". "ungetch()"
; will return -1 to indicate an error if it is called two or more
; times without an intervening call to "getch()" or "getche()".
; (The push-back buffer is a word to allow it to represent the 256
; possible byte values [0-255], plus the empty state [-1].)
;
;*******************************************************************************

include version.inc
.xlist
include cmacros.inc
include os2dll.inc
.list

sBegin data
assumes ds,data
extrn __chbuf:word ; Push-back buffer (-1 == empty)
sEnd



sBegin code

assumes cs,code
assumes ds,data

page
;***
;int _ungetch(c) - push back one character for "getch()" or "getche()"
;
;Purpose:
; If the Push-back buffer "__chbuf" is -1 Then
; Set "__chbuf" to the argument and return the argument
; Else
; Return -1 to indicate an error
;
;Entry:
; int c - Character to be pushed back
;
;Exit:
; If successful
; AX = character that was pushed back
; Else if error
; AX = -1
;
;Uses:
; DX
;
;Exceptions:
;
;*******************************************************************************

cProc _ungetch,,<>

parmW byt

cBegin

ungetch0: ; common code
mov dx,-1 ; -1 == EOF
mov ax,[__chbuf] ; see if the pushback buffer is empty
or ah,ah
jz err ; there is already a character pushed back

mov ax,byt
cmp ax,dx ; byt == EOF is not allowed
je done
mov dx,ax ; DX = byt for return value
mov ah,0 ; byt & 0xFF
mov [__chbuf],ax ; unget the character
err:
xchg ax,dx ; AX = -1 for error, otherwise "byt"

done:
pop bp
ret
cEnd

sEnd

end