Microsoft (R) Macro Assembler Version 5.10 2/17/92 16:13:56
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;------------------------------------------------------------------------------
; Module Name: READAXIS.ASM
;
; Assembly language routine for reading joystick axis position
;
; Created: Feb 17th, 1992
; Author: Peter Wan
;
; Copyright (c) 1992 Advanced Gravis Computer Technology Limited
;
; Public Functions:
; get_loop_count
; read_axis
; Public Data:
; None
; General Description:
; This module contains functions to initialize itself and return
; joystick axis position.
;------------------------------------------------------------------------------
dosseg
.model small
;------------------------------------------------------------------------------
; Port address
;------------------------------------------------------------------------------
= 0201 GAMEPORT equ 201h ;gameport address
= 0040 TIMER0 equ 40h ;8253 counter timer channel 0 address
= 0043 TIMER_CONTROL equ 43h ;counter timer control register address
.data
0000 0000 max_loop_count dw ? ;loop count for 3 milliseconds
.code
;------------------------------------------------------------------------------
; get_loop_count
;
; The purpose of this routine is to determine how many iteration of the
; following instruction loop can be accomplished in 3 milliseconds. The number
; of iteration count will be different on computers running at different speed
; or computers with different processing unit. In the read joystick axis
; routine, it uses a similar program loop to determine joystick axis position
; and the count results from this routine will be used to limit axis read time
; up to 3 millesecond maximum. This routine must be called once at
; initialization time.
;
; Entry:
; None
; Returns:
; None
; Error Returns:
; None
Microsoft (R) Macro Assembler Version 5.10 2/17/92 16:13:56
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; Registers Preserved:
; BX,SI,DI,BP,DS,ES
; Registers Destroyed:
; AX,CX,DX
; Calls:
; None
;------------------------------------------------------------------------------
public _get_loop_count
0000 _get_loop_count proc
= [bp-2] init_count equ <[bp-2]>
0000 55 push bp
0001 8B EC mov bp,sp
0003 83 EC 02 sub sp,2 ;2 bytes of local variable
0006 C7 06 0000 R 0000 mov max_loop_count,0;use loop count of 16 to start with
000C BA 0201 mov dx,GAMEPORT ;gameport address
000F glc3:
000F B8 FF00 mov ax,0ff00h
0012 83 06 0000 R 10 add max_loop_count,16
0017 8B 0E 0000 R mov cx,max_loop_count
001B FA cli ;system interrupt must be disabled
001C E6 43 out TIMER_CONTROL,al;latch count in timer 0
001E EB 00 jmp short $+2
0020 E4 40 in al,TIMER0 ;read low byte of latched count
0022 88 46 FE mov init_count,al ;save
0025 EB 00 jmp short $+2
0027 E4 40 in al,TIMER0 ;read high byte
0029 88 46 FF mov init_count+1,al ;save
002C EE out dx,al ;trigger gameport
002D EC in al,dx ;delay to allow gameport to switch on
002E glc4: ;the following loop exits only when CX
;reaches zero
002E EC in al,dx
002F 0A C4 or al,ah
0031 E0 FB loopnz glc4
0033 B8 0000 mov ax,0
0036 75 00 jnz $+2
0038 E6 43 out TIMER_CONTROL,al;latch count in timer 0
003A FB sti ;enable system interrupt
003B EB 00 jmp short $+2
003D E4 40 in al,TIMER0 ;read low byte of latched count
003F 8A E0 mov ah,al
0041 EB 00 jmp short $+2
0043 E4 40 in al,TIMER0 ;read high byte
0045 86 C4 xchg al,ah
0047 2B 46 FE sub ax,init_count ;subtract count latched before
;entering loop by count just read
004A F7 D8 neg ax
004C 3D 1BF8 cmp ax,7160 ;has it exceed 3 milliseconds?
004F 7C BE jl glc3 ;no, increment loop count by 16 and
;repeat process
0051 8B E5 mov sp,bp
0053 5D pop bp
0054 C3 ret
Microsoft (R) Macro Assembler Version 5.10 2/17/92 16:13:56
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0055 _get_loop_count endp
;------------------------------------------------------------------------------
; _read_axis
;
; This routine will accept a number from 0 to 3 for returning joystick axis
; position for joystick A-X, A-Y, B-X, B-Y respectively. After bit position
; of requested axis is determined, count reading in counter timer chip channel 0
; will be read and saved. A write cycle to gameport address 201 hex will
; trigger and toggle all four axis lines from low to high state. The duration
; of these individual line staying high is directly proportional to the
; position of joystick for the individual axis. After the requested axis line
; has returned to the low state, count reading in counter timer chip will be
; read again. The real time count of the duration the requested axis has stayed
; high can be obtained from the difference of the two counts read. Allowance
; for other axis to return to low state must also be provided before next read.
; Therefore, return to calling function can only be done at the end of the
; 3 milliseconds duration. The real time count return to the callind function
; can have a value ranges from 1 to maximum of 7160 depending on the position
; of the joystick, value of capacitors used and threshold voltage set
; on the gamecard. If the requested axis has not returned to the low state
; after 3 milliseconds, it is assumed that no joystick is connected to that
; axis and zero will be returned to the calling function. During the
; 3 milliseconds duration, all interrupts except NMI will be disabled.
;
; Entry:
; [bp+4] = Axis to be read
; 0 = Joystick A - X axis
; 1 = Joystick A - Y axis
; 2 = Joystick B - X axis
; 3 = Joystick B - Y axis
; Returns:
; AX = Count ranges from 1 to maximun of 7160 corresponds to joystick
; position
; Error Returns:
; AX = 0 if no joystick connected
; Registers Preserved:
; BX,SI,DI,BP,DS,ES
; Registers Destroyed:
; AX,CX,DX
; Calls:
; None
;------------------------------------------------------------------------------
public _read_axis
0055 _read_axis proc
= [bp+4] axis equ <[bp+4]>
= [bp-2] init_count equ <[bp-2]>
0055 55 push bp
0056 8B EC mov bp,sp
0058 83 EC 02 sub sp,2 ;2 bytes of local variable
005B 2B C0 sub ax,ax ;determine bit position of axis to be
005D B4 01 mov ah,1 ;read
005F 8B 4E 04 mov cx,axis
Microsoft (R) Macro Assembler Version 5.10 2/17/92 16:13:56
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0062 D2 E4 shl ah,cl
0064 8B 0E 0000 R mov cx,max_loop_count;use 3 milliseconds loop count obtian
;at initailization
0068 BA 0201 mov dx,GAMEPORT ;gameport address
006B FA cli ;disable interrupt
006C E6 43 out TIMER_CONTROL,al;latch count in timer 0
006E EB 00 jmp short $+2
0070 E4 40 in al,TIMER0 ;read low byte of latched count
0072 88 46 FE mov init_count,al ;save
0075 EB 00 jmp short $+2
0077 E4 40 in al,TIMER0 ;read high byte
0079 88 46 FF mov init_count+1,al ;save
007C EE out dx,al ;trigger gameport
007D EC in al,dx ;delay to allow gameport to switch on
007E ra1:
007E EC in al,dx ;read gameport
007F 84 C4 test al,ah ;requested axis return to low state or
;3 milliseconds passed?
0081 E0 FB loopnz ra1 ;no
0083 B8 0000 mov ax,0 ;return 0 if 3 milliseconds passed and
0086 75 1E jnz ra3 ;requested axis not yet in low state
0088 E6 43 out TIMER_CONTROL,al;requested axis return to low state
;latch count in timer 0
008A EB 00 jmp short $+2
008C E4 40 in al,TIMER0 ;read low byte of latched count
008E 8A E0 mov ah,al
0090 EB 00 jmp short $+2
0092 E4 40 in al,TIMER0 ;read high byte
0094 86 C4 xchg al,ah
0096 2B 46 FE sub ax,init_count ;obtain real time count of high state
0099 F7 D8 neg ax ;duration
009B E3 09 jcxz ra3 ;requested axis return to low state in
;exactly 3 milliseconds, all other axis
;should also have returned to low state
;by now
009D 50 push ax ;even though the requested axis has
009E B4 FF mov ah,0ffh ;returned to the low state, other axis
00A0 ra2: ;may still be in the high state, we
00A0 EC in al,dx ;should wait till the end of the
00A1 0A C4 or al,ah ;3 milliseconds duration to allow the
00A3 E0 FB loopnz ra2 ;other axis to return to high state
00A5 58 pop ax ;before returning to the calling
;fucntion
00A6 ra3:
00A6 FB sti ;enable system interrupt
00A7 8B E5 mov sp,bp
00A9 5D pop bp
00AA C3 ret
00AB _read_axis endp
end
Microsoft (R) Macro Assembler Version 5.10 2/17/92 16:13:56
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212 Source Lines
212 Total Lines
33 Symbols
47362 + 421160 Bytes symbol space free
0 Warning Errors
0 Severe Errors
