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Power-Programmierung
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CD1.mdf
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assemblr
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lst11_17.asm
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Assembly Source File
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1990-02-15
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3KB
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110 lines
;
; *** Listing 11-17 ***
;
; Demonstrates the calculation of the offset of the word
; matching a keystroke in a look-up table when SCASW is
; used, where the 2-byte overrun of SCASW must be
; compensated for. The offset in the look-up table is used
; to look up the corresponding address in a second table;
; that address is then jumped to in order to handle the
; keystroke.
;
; This is a standalone program, not to be used with PZTIME
; but rather assembled, linked, and run by itself.
;
stack segment para stack 'STACK'
db 512 dup (?)
stack ends
;
code segment para public 'CODE'
assume cs:code, ds:nothing
;
; Main loop, which simply calls VectorOnKey until one of the
; key handlers ends the program.
;
start proc near
call VectorOnKey
jmp start
start endp
;
; Gets the next 16-bit key code from the BIOS, looks it up
; in KeyLookUpTable, and jumps to the corresponding routine
; according to KeyJumpTable. When the jumped-to routine
; returns, is will return to the code that called
; VectorOnKey. Ignores the key if the key code is not in the
; look-up table.
;
; Input: none
;
; Output: none
;
; Registers altered: AX, CX, DI, ES
;
; Direction flag cleared
;
; Table of 16-bit key codes this routine handles.
;
KeyLookUpTable label word
dw 0011bh ;Esc to exit
dw 01c0dh ;Enter to beep
;*** Additional key codes go here ***
KEY_LOOK_UP_TABLE_LENGTH_IN_WORDS equ (($-KeyLookUpTable)/2)
;
; Table of addresses to jump to when corresponding key codes
; in KeyLookUpTable are found.
;
KeyJumpTable label word
dw EscHandler
dw EnterHandler
;*** Additional addresses go here ***
;
VectorOnKey proc near
WaitKeyLoop:
mov ah,1 ;BIOS key status function
int 16h ;invoke BIOS to see if
; a key is pending
jz WaitKeyLoop ;wait until key comes along
sub ah,ah ;BIOS get key function
int 16h ;invoke BIOS to get the key
push cs
pop es
mov di,offset KeyLookUpTable
;point ES:DI to the table of keys
; we handle, which is in the same
; segment as this code
mov cx,KEY_LOOK_UP_TABLE_LENGTH_IN_WORDS
;# of words to scan
cld
repnz scasw ;look up the key
jnz WaitKeyLoop ;it's not in the table, so
; ignore it
jmp cs:[KeyJumpTable+di-2-offset KeyLookUpTable]
;jump to the routine for this key
; Note that:
; DI-2-offset KeyLookUpTable
; is the offset in KeyLookUpTable of
; the key we found, with the -2
; needed to compensate for the
; 2-byte (1-word) overrun of SCASW
VectorOnKey endp
;
; Code to handle Esc (ends the program).
;
EscHandler proc near
mov ah,4ch ;DOS terminate program function
int 21h ;exit program
EscHandler endp
;
; Code to handle Enter (beeps the speaker).
;
EnterHandler proc near
mov ax,0e07h ;AH=0E is BIOS print character
; function, AL=7 is bell (beep)
; character
int 10h ;tell BIOS to beep the speaker
ret
EnterHandler endp
;
code ends
end start