Graphics Programming Black Book (Special Edition)/BlackBook.bin/disk1/source/chapter3-p1/pztimer.asm

Assembly Source File | 1997-06-18 | 11KB | 443 lines

;
; *** Listing 3-1 ***
;
; The precision Zen timer (PZTIMER.ASM)
;
; Uses the 8253 timer to time the performance of code that takes
; less than about 54 milliseconds to execute, with a resolution
; of better than 10 microseconds.
;
; By Michael Abrash
;
; Externally callable routines:
;
;  ZTimerOn: Starts the Zen timer, with interrupts disabled.
;
;  ZTimerOff: Stops the Zen timer, saves the timer count,
;    times the overhead code, and restores interrupts to the
;    state they were in when ZTimerOn was called.
;
;  ZTimerReport: Prints the net time that passed between starting
;    and stopping the timer.
;
; Note: If longer than about 54 ms passes between ZTimerOn and
;    ZTimerOff calls, the timer turns over and the count is
;    inaccurate. When this happens, an error message is displayed
;    instead of a count. The long-period Zen timer should be used
;    in such cases.
;
; Note: Interrupts *MUST* be left off between calls to ZTimerOn
;    and ZTimerOff for accurate timing and for detection of
;    timer overflow.
;
; Note: These routines can introduce slight inaccuracies into the
;    system clock count for each code section timed even if
;    timer 0 doesn't overflow. If timer 0 does overflow, the
;    system clock can become slow by virtually any amount of
;    time, since the system clock can't advance while the
;    precison timer is timing. Consequently, it's a good idea
;    to reboot at the end of each timing session. (The
;    battery-backed clock, if any, is not affected by the Zen
;    timer.)
;
; All registers, and all flags except the interrupt flag, are
; preserved by all routines. Interrupts are enabled and then disabled
; by ZTimerOn, and are restored by ZTimerOff to the state they were
; in when ZTimerOn was called.
;

Code    segment word public 'CODE'
    assume    cs:Code, ds:nothing
    public    ZTimerOn, ZTimerOff, ZTimerReport

;
; Base address of the 8253 timer chip.
;
BASE_8253        equ    40h
;
; The address of the timer 0 count registers in the 8253.
;
TIMER_0_8253        equ    BASE_8253 + 0
;
; The address of the mode register in the 8253.
;
MODE_8253        equ    BASE_8253 + 3
;
; The address of Operation Command Word 3 in the 8259 Programmable
; Interrupt Controller (PIC) (write only, and writable only when
; bit 4 of the byte written to this address is 0 and bit 3 is 1).
;
OCW3            equ    20h
;
; The address of the Interrupt Request register in the 8259 PIC
; (read only, and readable only when bit 1 of OCW3 = 1 and bit 0
; of OCW3 = 0).
;
IRR            equ    20h
;
; Macro to emulate a POPF instruction in order to fix the bug in some
; 80286 chips which allows interrupts to occur during a POPF even when
; interrupts remain disabled.
;
MPOPF macro
    local    p1, p2
    jmp short p2
p1:    iret            ;jump to pushed address & pop flags
p2:    push    cs        ;construct far return address to
    call    p1        ; the next instruction
    endm

;
; Macro to delay briefly to ensure that enough time has elapsed
; between successive I/O accesses so that the device being accessed
; can respond to both accesses even on a very fast PC.
;
DELAY    macro
    jmp    $+2
    jmp    $+2
    jmp    $+2
    endm

OriginalFlags        db    ?    ;storage for upper byte of
                    ; FLAGS register when
                    ; ZTimerOn called
TimedCount        dw    ?    ;timer 0 count when the timer
                    ; is stopped
ReferenceCount        dw    ?    ;number of counts required to
                    ; execute timer overhead code
OverflowFlag        db    ?    ;used to indicate whether the
                    ; timer overflowed during the
                    ; timing interval
;
; String printed to report results.
;
OutputStr    label    byte
        db    0dh, 0ah, 'Timed count: ', 5 dup (?)
ASCIICountEnd    label    byte
        db    ' microseconds', 0dh, 0ah
        db    '$'
;
; String printed to report timer overflow.
;
OverflowStr    label    byte
    db    0dh, 0ah
    db    '****************************************************'
    db    0dh, 0ah
    db    '* The timer overflowed, so the interval timed was  *'
    db    0dh, 0ah
    db    '* too long for the precision timer to measure.     *'
    db    0dh, 0ah
    db    '* Please perform the timing test again with the    *'
    db    0dh, 0ah
    db    '* long-period timer.                               *'
    db    0dh, 0ah
    db    '****************************************************'
    db    0dh, 0ah
    db    '$'

;********************************************************************
;* Routine called to start timing.                    *
;********************************************************************

ZTimerOn    proc    near

;
; Save the context of the program being timed.
;
    push    ax
    pushf
    pop    ax            ;get flags so we can keep
                    ; interrupts off when leaving
                    ; this routine
    mov    cs:[OriginalFlags],ah    ;remember the state of the
                    ; Interrupt flag
    and    ah,0fdh         ;set pushed interrupt flag
                    ; to 0
    push    ax
;
; Turn on interrupts, so the timer interrupt can occur if it's
; pending.
;
    sti
;
; Set timer 0 of the 8253 to mode 2 (divide-by-N), to cause
; linear counting rather than count-by-two counting. Also
; leaves the 8253 waiting for the initial timer 0 count to
; be loaded.
;
    mov    al,00110100b        ;mode 2
    out    MODE_8253,al
;
; Set the timer count to 0, so we know we won't get another
; timer interrupt right away.
; Note: this introduces an inaccuracy of up to 54 ms in the system
; clock count each time it is executed.
;
    DELAY
    sub    al,al
    out    TIMER_0_8253,al        ;lsb
    DELAY
    out    TIMER_0_8253,al        ;msb
;
; Wait before clearing interrupts to allow the interrupt generated
; when switching from mode 3 to mode 2 to be recognized. The delay
; must be at least 210 ns long to allow time for that interrupt to
; occur. Here, 10 jumps are used for the delay to ensure that the
; delay time will be more than long enough even on a very fast PC.
;
    rept 10
    jmp    $+2
    endm
;
; Disable interrupts to get an accurate count.
;
    cli
;
; Set the timer count to 0 again to start the timing interval.
;
    mov    al,00110100b        ;set up to load initial
    out    MODE_8253,al        ; timer count
    DELAY
    sub    al,al
    out    TIMER_0_8253,al        ;load count lsb
    DELAY
    out    TIMER_0_8253,al        ;load count msb
;
; Restore the context and return.
;
    MPOPF                ;keeps interrupts off
    pop    ax
    ret

ZTimerOn    endp

;********************************************************************
;* Routine called to stop timing and get count.                *
;********************************************************************

ZTimerOff proc    near

;
; Save the context of the program being timed.
;
    push    ax
    push    cx
    pushf
;
; Latch the count.
;
    mov    al,00000000b        ;latch timer 0
    out    MODE_8253,al
;
; See if the timer has overflowed by checking the 8259 for a pending
; timer interrupt.
;
    mov    al,00001010b        ;OCW3, set up to read
    out    OCW3,al            ; Interrupt Request register
    DELAY
    in    al,IRR            ;read Interrupt Request
                    ; register
    and    al,1            ;set AL to 1 if IRQ0 (the
                    ; timer interrupt) is pending
    mov    cs:[OverflowFlag],al    ;store the timer overflow
                    ; status
;
; Allow interrupts to happen again.
;
    sti
;
; Read out the count we latched earlier.
;
    in    al,TIMER_0_8253        ;least significant byte
    DELAY
    mov    ah,al
    in    al,TIMER_0_8253        ;most significant byte
    xchg    ah,al
    neg    ax            ;convert from countdown
                    ; remaining to elapsed
                    ; count
    mov    cs:[TimedCount],ax
; Time a zero-length code fragment, to get a reference for how
; much overhead this routine has. Time it 16 times and average it,
; for accuracy, rounding the result.
;
    mov    cs:[ReferenceCount],0
    mov    cx,16
    cli                ;interrupts off to allow a
                    ; precise reference count
RefLoop:
    call    ReferenceZTimerOn
    call    ReferenceZTimerOff
    loop    RefLoop
    sti
    add    cs:[ReferenceCount],8    ;total + (0.5 * 16)
    mov    cl,4
    shr    cs:[ReferenceCount],cl    ;(total) / 16 + 0.5
;
; Restore original interrupt state.
;
    pop    ax            ;retrieve flags when called
    mov    ch,cs:[OriginalFlags]    ;get back the original upper
                    ; byte of the FLAGS register
    and    ch,not 0fdh        ;only care about original
                    ; interrupt flag...
    and    ah,0fdh            ;...keep all other flags in
                    ; their current condition
    or    ah,ch            ;make flags word with original
                    ; interrupt flag
    push    ax            ;prepare flags to be popped
;
; Restore the context of the program being timed and return to it.
;
    MPOPF                ;restore the flags with the
                    ; original interrupt state
    pop    cx
    pop    ax
    ret

ZTimerOff endp

;
; Called by ZTimerOff to start timer for overhead measurements.
;

ReferenceZTimerOn    proc    near
;
; Save the context of the program being timed.
;
    push    ax
    pushf        ;interrupts are already off
;
; Set timer 0 of the 8253 to mode 2 (divide-by-N), to cause
; linear counting rather than count-by-two counting.
;
    mov    al,00110100b    ;set up to load
    out    MODE_8253,al    ; initial timer count
    DELAY
;
; Set the timer count to 0.
;
    sub    al,al
    out    TIMER_0_8253,al    ;load count lsb
    DELAY
    out    TIMER_0_8253,al    ;load count msb
;
; Restore the context of the program being timed and return to it.
;
    MPOPF
    pop    ax
    ret

ReferenceZTimerOn    endp

;
; Called by ZTimerOff to stop timer and add result to ReferenceCount
; for overhead measurements.
;

ReferenceZTimerOff proc    near
;
; Save the context of the program being timed.
;
    push    ax
    push    cx
    pushf
;
; Latch the count and read it.
;
    mov    al,00000000b        ;latch timer 0
    out    MODE_8253,al
    DELAY
    in    al,TIMER_0_8253        ;lsb
    DELAY
    mov    ah,al
    in    al,TIMER_0_8253        ;msb
    xchg    ah,al
    neg    ax            ;convert from countdown
                    ; remaining to amount
                    ; counted down
    add    cs:[ReferenceCount],ax
;
; Restore the context of the program being timed and return to it.
;
    MPOPF
    pop    cx
    pop    ax
    ret

ReferenceZTimerOff endp

;********************************************************************
;* Routine called to report timing results.                *
;********************************************************************

ZTimerReport    proc    near

    pushf
    push    ax
    push    bx
    push    cx
    push    dx
    push    si
    push    ds
;
    push    cs    ;DOS functions require that DS point
    pop    ds    ; to text to be displayed on the screen
    assume    ds:Code
;
; Check for timer 0 overflow.
;
    cmp    [OverflowFlag],0
    jz    PrintGoodCount
    mov    dx,offset OverflowStr
    mov    ah,9
    int    21h
    jmp    short EndZTimerReport
;
; Convert net count to decimal ASCII in microseconds.
;
PrintGoodCount:
    mov    ax,[TimedCount]
    sub    ax,[ReferenceCount]
    mov    si,offset ASCIICountEnd - 1
;
; Convert count to microseconds by multiplying by .8381.
;
    mov    dx,8381
    mul    dx
    mov    bx,10000
    div    bx        ;* .8381 = * 8381 / 10000
;
; Convert time in microseconds to 5 decimal ASCII digits.
;
    mov    bx,10
    mov    cx,5
CTSLoop:
    sub    dx,dx
    div    bx
    add    dl,'0'
    mov    [si],dl
    dec    si
    loop    CTSLoop
;
; Print the results.
;
    mov    ah,9
    mov    dx,offset OutputStr
    int    21h
;
EndZTimerReport:
    pop    ds
    pop    si
    pop    dx
    pop    cx
    pop    bx
    pop    ax
    MPOPF
    ret

ZTimerReport    endp

Code    ends
    end