Category : Pascal Source Code
Archive   : TIMERASM.ZIP
Filename : LZTIME.ASM

;
; *** Listing 2-5 ***
;
; The long-period Zen timer. (LZTIMER.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 4/26/89
;
; 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.
; When the PS/2 version is used, each block of code being timed
; should be run several times, with at least two similar
; readings required to establish a true measurement.
;
; Note: When PS2 is 0, the code relies on an undocumented 8253
; feature. 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: 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.
;

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

;
; Set to 0 to assemble for use on a fully 8253-compatible
; system. Set to 1 to assemble for use on non-8253-compatible
; systems, including PS/2 computers. In general, leave this
; set to 0 on non-PS/2 computers unless you get inconsistent
; or inaccurate readings.
;
PS2 equ 0
;
; 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:Code
;
; 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

Code ends
end