Graphics Programming Black Book (Special Edition)/BlackBook.bin/disk1/source/chapterk/pcltnear.asm

Assembly Source File | 1997-06-18 | 16.6 KB | 639 lines

;
; *** Listing A ***
; The C-near-callable version of the long-period Zen timer
;       (LZTIMER.ASM)
;
; The long-period Zen timer. (PCLTNEAR.ASM)
; Uses the 8253 timer and the BIOS time-of-day count to time the
; performance of code that takes less than an hour to execute.
; Because interrupts are left on (in order to allow the timer
; interrupt to be recognized), this is less accurate than the
; precision Zen timer, so it is best used only to time code that takes
; more than about 54 milliseconds to execute (code that the precision
; Zen timer reports overflow on). Resolution is limited by the
; occurrence of timer interrupts.
;
; By Michael Abrash
;
; Externally callable routines:
;
;  ZTimerOn: Saves the BIOS time of day count and starts the
;    long-period Zen timer.
;
;  ZTimerOff: Stops the long-period Zen timer and saves the timer
;    count and the BIOS time-of-day count.
;
;  ZTimerReport: Prints the time that passed between starting and
;    stopping the timer.
;
; Note: If either more than an hour passes or midnight falls between
;    calls to ZTimerOn and ZTimerOff, an error is reported. For
;    timing code that takes more than a few minutes to execute,
;    either the DOS TIME command in a batch file before and after
;    execution of the code to time or the use of the DOS
;    time-of-day function in place of the long-period Zen timer is
;    more than adequate.
;
; Note: The PS/2 version is assembled by setting the symbol PS2 to 1.
;    PS2 must be set to 1 on PS/2 computers because the PS/2's
;    timers are not compatible with an undocumented timer-stopping
;    feature of the 8253; the alternative timing approach that
;    must be used on PS/2 computers leaves a short window
;    during which the timer 0 count and the BIOS timer count may
;    not be synchronized. You should also set the PS2 symbol to
;    1 if you're getting erratic or obviously incorrect results.
;
; Note: When PS2 is 0, the code relies on an undocumented 8253
;    feature to get more reliable readings. It is possible that
;    the 8253 (or whatever chip is emulating the 8253) may be put
;    into an undefined or incorrect state when this feature is
;    used.
;
;     ***************************************************************
;     * If your computer displays any hint of erratic behavior        *
;     *    after the long-period Zen timer is used, such as the floppy *
;     *    drive failing to operate properly, reboot the system, set   *
;     *    PS2 to 1 and leave it that way!                    *
;     ***************************************************************
;
; Note: Each block of code being timed should ideally be run several
;    times, with at least two similar readings required to
;    establish a true measurement, in order to eliminate any
;    variability caused by interrupts.
;
; Note: Interrupts must not be disabled for more than 54 ms at a
;    stretch during the timing interval. Because interrupts
;    are enabled, keys, mice, and other devices that generate
;    interrupts should not be used during the timing interval.
;
; Note: Any extra code running off the timer interrupt (such as
;    some memory-resident utilities) will increase the time
;    measured by the Zen timer.
;
; Note: These routines can introduce inaccuracies of up to a few
;    tenths of a second into the system clock count for each
;    code section timed. Consequently, it's a good idea to
;    reboot at the conclusion of timing sessions. (The
;    battery-backed clock, if any, is not affected by the Zen
;    timer.)
;
; All registers and all flags are preserved by all routines.
;

_TEXT    segment word public 'CODE'
    assume    cs:_TEXT, ds:nothing
    public    _ZTimerOn, _ZTimerOff, _ZTimerReport

;
; Set PS2 to 0 to assemble for use on a fully 8253-compatible
; system; when PS2 is 0, the readings are more reliable if the
; computer supports the undocumented timer-stopping feature,
; but may be badly off if that feature is not supported. In
; fact, timer-stopping may interfere with your computer's
; overall operation by putting the 8253 into an undefined or
; incorrect state.  Use with caution!!!
;
; Set PS2 to 1 to assemble for use on non-8253-compatible
; systems, including PS/2 computers; when PS2 is 1, readings
; may occasionally be off by 54 ms, but the code will work
; properly on all systems.
;
; A setting of 1 is safer and will work on more systems,
; while a setting of 0 produces more reliable results in systems
; which support the undocumented timer-stopping feature of the
; 8253. The choice is yours.
;
PS2    equ    1
;
; 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 the BIOS timer count variable in the BIOS
; data segment.
;
TIMER_COUNT        equ    46ch
;
; 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

StartBIOSCountLow    dw    ?    ;BIOS count low word at the
                    ; start of the timing period
StartBIOSCountHigh    dw    ?    ;BIOS count high word at the
                    ; start of the timing period
EndBIOSCountLow        dw    ?    ;BIOS count low word at the
                    ; end of the timing period
EndBIOSCountHigh    dw    ?    ;BIOS count high word at the
                    ; end of the timing period
EndTimedCount        dw    ?    ;timer 0 count at the end of
                    ; the timing period
ReferenceCount        dw    ?    ;number of counts required to
                    ; execute timer overhead code
;
; String printed to report results.
;
OutputStr    label    byte
        db    0dh, 0ah, 'Timed count: '
TimedCountStr    db    10 dup (?)
        db    ' microseconds', 0dh, 0ah
        db    '$'
;
; Temporary storage for timed count as it's divided down by powers
; of ten when converting from doubleword binary to ASCII.
;
CurrentCountLow        dw    ?
CurrentCountHigh    dw    ?
;
; Powers of ten table used to perform division by 10 when doing
; doubleword conversion from binary to ASCII.
;
PowersOfTen    label    word
    dd    1
    dd    10
    dd    100
    dd    1000
    dd    10000
    dd    100000
    dd    1000000
    dd    10000000
    dd    100000000
    dd    1000000000
PowersOfTenEnd    label    word
;
; String printed to report that the high word of the BIOS count
; changed while timing (an hour elapsed or midnight was crossed),
; and so the count is invalid and the test needs to be rerun.
;
TurnOverStr    label    byte
    db    0dh, 0ah
    db    '****************************************************'
    db    0dh, 0ah
    db    '* Either midnight passed or an hour or more passed *'
    db    0dh, 0ah
    db    '* while timing was in progress. If the former was  *'
    db    0dh, 0ah
    db    '* the case, please rerun the test; if the latter   *'
    db    0dh, 0ah
    db    '* was the case, the test code takes too long to    *'
    db    0dh, 0ah
    db    '* run to be timed by the long-period Zen timer.    *'
    db    0dh, 0ah
    db    '* Suggestions: use the DOS TIME command, the DOS   *'
    db    0dh, 0ah
    db    '* time function, or a watch.                       *'
    db    0dh, 0ah
    db    '****************************************************'
    db    0dh, 0ah
    db    '$'

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

_ZTimerOn    proc    near

;
; Save the context of the program being timed.
;
    push    ax
    pushf
;
; Set timer 0 of the 8253 to mode 2 (divide-by-N), to cause
; linear counting rather than count-by-two counting. Also stops
; timer 0 until the timer count is loaded, except on PS/2
; computers.
;
    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
;
; In case interrupts are disabled, enable interrupts briefly to allow
; the interrupt generated when switching from mode 3 to mode 2 to be
; recognized. Interrupts must be enabled for at least 210 ns 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.
;
    pushf
    sti
    rept 10
    jmp    $+2
    endm
    MPOPF
;
; Store the timing start BIOS count.
; (Since the timer count was just set to 0, the BIOS count will
; stay the same for the next 54 ms, so we don't need to disable
; interrupts in order to avoid getting a half-changed count.)
;
    push    ds
    sub    ax,ax
    mov    ds,ax
    mov    ax,ds:[TIMER_COUNT+2]
    mov    cs:[StartBIOSCountHigh],ax
    mov    ax,ds:[TIMER_COUNT]
    mov    cs:[StartBIOSCountLow],ax
    pop    ds
;
; 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 of the program being timed and return to it.
;
    MPOPF
    pop    ax
    ret

_ZTimerOn    endp

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

_ZTimerOff proc    near

;
; Save the context of the program being timed.
;
    pushf
    push    ax
    push    cx
;
; In case interrupts are disabled, enable interrupts briefly to allow
; any pending timer interrupt to be handled. Interrupts must be
; enabled for at least 210 ns 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.
;
    sti
    rept    10
    jmp    $+2
    endm

;
; Latch the timer count.
;

if PS2

    mov    al,00000000b
    out    MODE_8253,al        ;latch timer 0 count
;
; This is where a one-instruction-long window exists on the PS/2.
; The timer count and the BIOS count can lose synchronization;
; since the timer keeps counting after it's latched, it can turn
; over right after it's latched and cause the BIOS count to turn
; over before interrupts are disabled, leaving us with the timer
; count from before the timer turned over coupled with the BIOS
; count from after the timer turned over. The result is a count
; that's 54 ms too long.
;

else

;
; Set timer 0 to mode 2 (divide-by-N), waiting for a 2-byte count
; load, which stops timer 0 until the count is loaded. (Only works
; on fully 8253-compatible chips.)
;
    mov    al,00110100b        ;mode 2
    out    MODE_8253,al
    DELAY
    mov    al,00000000b        ;latch timer 0 count
    out    MODE_8253,al

endif

    cli                ;stop the BIOS count
;
; Read the BIOS count. (Since interrupts are disabled, the BIOS
; count won't change.)
;
    push    ds
    sub    ax,ax
    mov    ds,ax
    mov    ax,ds:[TIMER_COUNT+2]
    mov    cs:[EndBIOSCountHigh],ax
    mov    ax,ds:[TIMER_COUNT]
    mov    cs:[EndBIOSCountLow],ax
    pop    ds
;
; Read the timer count and save it.
;
    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 elapsed
                    ; count
    mov    cs:[EndTimedCount],ax
;
; Restart timer 0, which is still waiting for an initial count
; to be loaded.
;

ife PS2

    DELAY
    mov    al,00110100b        ;mode 2, waiting to load a
                    ; 2-byte count
    out    MODE_8253,al
    DELAY
    sub    al,al
    out    TIMER_0_8253,al        ;lsb
    DELAY
    mov    al,ah
    out    TIMER_0_8253,al        ;msb
    DELAY

endif

    sti        ;let the BIOS count continue
;
; 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 the context of the program being timed and return to it.
;
    pop    cx
    pop    ax
    MPOPF
    ret

_ZTimerOff endp

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

ReferenceZTimerOn    proc    near
;
; Save the context of the program being timed.
;
    push    ax
    pushf
;
; 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    ;mode 2
    out    MODE_8253,al
;
; Set the timer count to 0.
;
    DELAY
    sub    al,al
    out    TIMER_0_8253,al        ;lsb
    DELAY
    out    TIMER_0_8253,al        ;msb
;
; Restore the context of the program being timed and return to it.
;
    MPOPF
    pop    ax
    ret

ReferenceZTimerOn    endp

;
; Called by ZTimerOff to stop the timer and add the result to
; ReferenceCount for overhead measurements. Doesn't need to look
; at the BIOS count because timing a zero-length code fragment
; isn't going to take anywhere near 54 ms.
;

ReferenceZTimerOff proc    near
;
; Save the context of the program being timed.
;
    pushf
    push    ax
    push    cx

;
; Match the interrupt-window delay in ZTimerOff.
;
    sti
    rept    10
    jmp    $+2
    endm

    mov    al,00000000b
    out    MODE_8253,al        ;latch timer
;
; Read the count and save it.
;
    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 elapsed
                    ; count
    add    cs:[ReferenceCount],ax
;
; Restore the context and return.
;
    pop    cx
    pop    ax
    MPOPF
    ret

ReferenceZTimerOff endp

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

_ZTimerReport    proc    near

    pushf
    push    ax
    push    bx
    push    cx
    push    dx
    push    si
    push    di
    push    ds
;
    push    cs    ;DOS functions require that DS point
    pop    ds    ; to text to be displayed on the screen
    assume    ds:_TEXT
;
; See if midnight or more than an hour passed during timing. If so,
; notify the user.
;
    mov    ax,[StartBIOSCountHigh]
    cmp    ax,[EndBIOSCountHigh]
    jz    CalcBIOSTime        ;hour count didn't change,
                    ; so everything's fine
    inc    ax
    cmp    ax,[EndBIOSCountHigh]
    jnz    TestTooLong        ;midnight or two hour
                    ; boundaries passed, so the
                    ; results are no good
    mov    ax,[EndBIOSCountLow]
    cmp    ax,[StartBIOSCountLow]
    jb    CalcBIOSTime        ;a single hour boundary
                    ; passed-that's OK, so long as
                    ; the total time wasn't more
                    ; than an hour

;
; Over an hour elapsed or midnight passed during timing, which
; renders the results invalid. Notify the user. This misses the
; case where a multiple of 24 hours has passed, but we'll rely
; on the perspicacity of the user to detect that case.
;
TestTooLong:
    mov    ah,9
    mov    dx,offset TurnOverStr
    int    21h
    jmp    short ZTimerReportDone
;
; Convert the BIOS time to microseconds.
;
CalcBIOSTime:
    mov    ax,[EndBIOSCountLow]
    sub    ax,[StartBIOSCountLow]
    mov    dx,54925        ;number of microseconds each
                    ; BIOS count represents
    mul    dx
    mov    bx,ax            ;set aside BIOS count in
    mov    cx,dx            ; microseconds
;
; Convert timer count to microseconds.
;
    mov    ax,[EndTimedCount]
    mov    si,8381
    mul    si
    mov    si,10000
    div    si        ;* .8381 = * 8381 / 10000
;
; Add timer and BIOS counts together to get an overall time in
; microseconds.
;
    add    bx,ax
    adc    cx,0
;
; Subtract the timer overhead and save the result.
;
    mov    ax,[ReferenceCount]
    mov    si,8381        ;convert the reference count
    mul    si        ; to microseconds
    mov    si,10000
    div    si        ;* .8381 = * 8381 / 10000
    sub    bx,ax
    sbb    cx,0
    mov    [CurrentCountLow],bx
    mov    [CurrentCountHigh],cx
;
; Convert the result to an ASCII string by trial subtractions of
; powers of 10.
;
    mov    di,offset PowersOfTenEnd - offset PowersOfTen - 4
    mov    si,offset TimedCountStr
CTSNextDigit:
    mov    bl,'0'
CTSLoop:
    mov    ax,[CurrentCountLow]
    mov    dx,[CurrentCountHigh]
    sub    ax,PowersOfTen[di]
    sbb    dx,PowersOfTen[di+2]
    jc    CTSNextPowerDown
    inc    bl
    mov    [CurrentCountLow],ax
    mov    [CurrentCountHigh],dx
    jmp    CTSLoop
CTSNextPowerDown:
    mov    [si],bl
    inc    si
    sub    di,4
    jns    CTSNextDigit
;
;
; Print the results.
;
    mov    ah,9
    mov    dx,offset OutputStr
    int    21h
;
ZTimerReportDone:
    pop    ds
    pop    di
    pop    si
    pop    dx
    pop    cx
    pop    bx
    pop    ax
    MPOPF
    ret

_ZTimerReport    endp

_TEXT    ends
    end