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141 lines
3.0 KiB
NASM
141 lines
3.0 KiB
NASM
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PAGE ,132
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title \asm_sour\timer.asm HIGH ACCURACY TIMER
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subttl michael e. walraven
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.MODEL MEDIUM
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name timer
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.cref
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.lall
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;
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; High resolution timer, returns a 32 bit high resolution
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; value which is the amount of elapsed time since the function
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; was last called. The counts are 838.2ns each (1.19318 MHz)
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; time_int() must be called first to set the timer chip to
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; the proper mode.
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; Counter 0 is changed in time_int() and the data from this
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; counter is used in elaptime() so it must not be changed
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; between calls.
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; There should not be any interference in system timing
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; max of 55 msec error introduced by time_int() into absolute
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; system time.
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; MEDIUM memory model/microsoft 5.00
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; FAR PROGRAM, NEAR DATA
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; cs: is code segment
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; es: and ds: are data segment
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; ss: within data segment
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; ax: for integer return
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; dx:ax: for long return
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.DATA
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; these data items located in the DSEG and can be accessed
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; as near by C programs
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PUBLIC SYS_HI
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PUBLIC SYS_LOW
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PUBLIC TIMER_COUNT
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SYS_HI DW ? ;TIMER_HI VALUE FOR PREVIOUS CALL
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SYS_LOW DW ? ;TIMER_LOW VALUE FOR PREVIOUS CALL
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TIMER_COUNT DW ? ;8253 TIMER COUNT FOR PREVIOUS CALL
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; NO ARGUMENTS PASSED to either function
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TIMER_MODE EQU 043H
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TIMER0 EQU 040H
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BIOS SEGMENT AT 040H
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ORG 06CH
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TIMER_LOW DW ?
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TIMER_HI DW ?
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BIOS ENDS
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PAGE
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.CODE
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PUBLIC _time_int
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_time_int PROC
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; void far time_int(void);
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;
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; SET THE TIMER MODE FOR PULSE OUTPUT, RATHER THAN SQUARE
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; MODE AS SET BY DOS
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MOV AL,00110100B ;CTR 0, LSB THEN MSB
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;MODE 2, BINARY
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OUT TIMER_MODE,AL ;MODE REGISTER FOR 8253
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SUB AX,AX ;SET 0, RESULT IN MAX COUNT
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OUT TIMER0,AL
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OUT TIMER0,AL
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RET
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_time_int ENDP
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PUBLIC _elaptime
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_elaptime PROC
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; long int far elaptime(void);
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;
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; DETERMINE ELAPSED TIME SINCE LAST CALL
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; RETURNS 32 BIT (LONG) VALUE WHICH IS
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; NEW - TIMER_HI:TIMER_LO:TIMER_COUNT MINUS
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; OLD - TIMER_HI:TIMER_LO:TIMER_COUNT
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; ASSUMPTION MADE THAT 32 BITS WILL NOT OVERFLOW!!!!
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PUSH ES
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MOV AX,BIOS
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MOV ES,AX
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ASSUME ES:BIOS
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MOV AL,0 ;PREPARE TO LATCH COUNTER
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OUT TIMER_MODE,AL ;LATCH 8253
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PUSHF ;SAVE INTERRUPT STATE
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CLI ;TURN INTERRUPT OFF WHILE READING CODE
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IN AL,TIMER0
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MOV DL,AL
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IN AL,TIMER0
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MOV DH,AL ;DX HAS NEW CHIP COUNT(count down value)
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MOV BX,ES:TIMER_LOW ;BX HAS SYSTEM TIME LOW WORD
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MOV AX,ES:TIMER_HI ;AX HAS SYSTEM TIME HIGH WORD
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; NOW HAVE A 48 BIT WORD AX:BX:DX FOR THE PRESENT TIME
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MOV CX,TIMER_COUNT ;SWAP AND SUBTRACT
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MOV TIMER_COUNT,DX
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SUB CX,DX
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; CX: HAS LOW 16 BITS OF DIFFERENCE
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MOV DX,SYS_LOW ;SWAP AND SUBTRACT
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MOV SYS_LOW,BX
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SBB BX,DX
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; BX: HAS MID 16 BITS OF DIFFERENCE
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MOV DX,SYS_HI ;SWAP AND SUBTRACT
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MOV SYS_HI,AX
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SBB AX,DX
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; AX: HAS HIGH 16 BITS OF DIFFERENCE
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; NOW HAVE A 48 BIT WORD THAT IS DIFFERENCE
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; ONLY PASS BACK 32 BITS AT PRESENT
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; AS DX:AX
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MOV AX,CX ;LOW 16 BITS
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MOV DX,BX ;MID 16 BITS
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POPF
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POP ES
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RET
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_elaptime ENDP
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END
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