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Using the 8254 Timer-Counter

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Using the 8254 Timer-Counter

Understanding the role of the system’s 8254 programmable Interval-Timer/Counter

- Algorithm steps:
- Get the count of timer-interrupts so far today
- Convert these ‘timer-ticks’ into seconds
- Breakdown the total number of seconds today into Hours, Minutes, Seconds, and AM/PM
- Convert numerical values into digit-strings
- Output these results to the video terminal

main memory

Number of timer-tick

interrupts so far today

(longword at 0x0046C)

0x00500

ROM-BIOS DATA AREA

0040:006C

tick_count

0x00400

Interrupt Vector Table

(for real-mode)

0x00000

- The ROM-BIOS interrupt-handler for the timer interrupt stores the tick-count as a 32-bit integer located at address 0x046C (it’s in the ROM-BIOS DATA AREA)
- In real-mode, we can get it like this:

xor%ax, %ax# address segment zero

mov%ax, %fs# using FS register

mov%fs:0x046C, %eax# copy tick-count to EAX

mov%eax, total_ticks# save in a local variable

segment-override prefix (segment used would be %ds)

total_ticks_today

total_seconds_today =

number of ticks-per-second

The number of ‘ticks-per-second’ is based upon the way

the PC’s timing hardware has been programmed

- The 8254 Programmable Interval-timer is used by the PC system for (1) generating timer-tick interrupts (rate is 18.2 per sec), (2) performing dynamic memory-refresh (reads ram once every 15 microseconds), and (3) generates ‘beeps’ of PC speaker
- When the speaker-function isn’t needed, the 8254 is available for other purposes

- The input-pulses to each Timer-channel is a long established PC standard, based on the design of the chrystal oscillator chip:1,193,182 pulses-per-second (Hertz)
- The frequency of the output-pulses from any Timer-channel is determined by how that channel’s Latch was programmed

8284

PCLK

1193182 Hz

Channel 0

CLK0

OUT0

Interrupt IRQ0

GATE0

Port 0x61, bit #4

CLK1

Channel 1

OUT1

DRAM refresh

GATE1

Port 0x61, bit #5

CLK2

Channel 2

OUT2

GATE2

speaker

AND

Port 0x61, bit #0

8254 PIT

+5 V

Port 0x61, bit #1

COUNT REGISTER

CLK

MSB

LSB

OUT

MSB

LSB

LATCH REGISTER

GATE

STATUS

TIMER/COUNTER CHANNEL

7 6 5 4 3 2 1 0

CHANNEL

COMMAND

OUTPUT MODE

binary

/ BCD

Output Mode

000 = one-shot level

001 = retriggerable

010 = rate-generator

011 = square-wave

100 = software strobe

101 = hardware strobe

Counting Mode

0 = binary

1 = BCD

Channel-ID

00 = chn 0

01 = chn 1

10 = chn 2

Command-ID

00 = Latch

01 = LSB r/w

10 = MSB r/w

11 = LSB-MSB r/w

Commands are sent to the 8254 via io/port 0x43

- Step 1: send command to PIT (port 0x43)
- Step 2: read or write the channel’s Latch
- via port 0x40 for channel 0
- via port 0x41 for channel 1
- via port 0x42 for channel 2

- For Channel 0 (the ‘timer-tick’ interrupt) the Latch is programmed during system startup with a value of zero
- But the Timer interprets zero as 65,536
- So the frequency of the output-pulses from Timer-channel 0 is equal to this quotient:
output-frequency = input-frequency / frequency-divisor

= 1193182 / 65536 (approximately 18.2)

- To compute ‘total_seconds’ from ‘total_ticks’:
total_seconds = total_ticks / ticks_per_second

= total_ticks / (1193182 / 65536)

= ( total_ticks * 65536 ) / 1193183

- We can use the Pentium’s integer-arithmetic instructions MUL (multiply) and DIV (divide)

Before executing the MUL instruction…

EAX

multiplicand (32-bits)

reg (or mem)

multiplier (32-bits)

32-bit operands

mull reg_or_mem

Here’s the instruction…

After executing the MUL instruction…

product (64-bits)

EDX

EAX

64-bit product

Before executing the DIV instruction…

dividend (64-bits)

EDX

EAX

64-bit dividend

reg (or mem)

divisor (32-bits)

32-bit operand

divl reg_or_mem

Here’s the instruction…

After executing the DIV instruction…

two results (32-bits)

EDX

EAX

32-bit remainder

32-bit quotient

- So use MUL and DIV to convert ‘ticks’ into ‘seconds’, like this:

# total_seconds = ( total_ticks * FREQ_DIVISOR ) / PULSES_PER_SEC

movtotal_ticks, %eax

mov$FREQ_DIVISOR, %ecx

mul%ecx

mov$PULSES_PER_SEC, %ecx

div%ecx

mov%eax, total_seconds

# Now integer-quotient is in EAX, and integer-remainder is in EDX

HH:MM:SS am/pm

hours

seconds

morning or

afternoon

minutes

So we need to compute four numerical values from the ‘total_seconds’ integer

We use these arithmetical ideas:

- total_minutes = ( total_seconds / 60 ); ss = ( total_seconds % 60 );
- total_hours = (total_minutes / 60 );mm = ( total_minutes % 60 );
- total_halfdays = (total_hours / 12 );hh = (total_hours % 12 );
- Total_days = ( total_halfdays / 2 );xm = total_halfdays % 2;

- Our ‘total_seconds’ value was gotten with an integer-division operation, so there’s likely to be some ‘round-off’ error
- How can we be sure we use the ‘closest’ integer to the actual quotient?
- We should remember the ‘rounding’ rule!
- When ‘remainder’ is equal or greater than 1/2 of ‘divisor’, ‘quotient’ gets incremented

- There is more than one way to do it – i.e., the “amateur’s” way or the “expert’s” way
- Knowledge of the Pentium’s architecture and instruction-set can assist
- The ‘obvious’ method:
- if ( 2 * remainder >= divisor ) ++quotient;

- Replace the ‘multiply’ with an ‘addition’
- Use ‘subtract’ and ‘add-with-carry’ instead of using ‘compare’ and ‘conditionally-jump’

# Recall: quotient was in EAX, remainder was in EDX, divisor was in ECX

add%edx, %edx# doubles the remainder

sub%ecx, %edx# computes: 2*quotient – divisor

# now carry-flag is clear in case 2*quotient >= divisor

cmc# complement the carry-flag bit

# now carry-flag is set in case 2*quotient >= divisor

adc$0, %eax# add the carry-flag to the quotient

# So this achieves the same effect as the ‘rounding rule’, but wit no jump!

- Can you enhance our ‘timeoday.s’ demo to make it more dramatic (and later useful) by creating a loop within its ‘main’ routine, so it continues to read and display the time (until the user presses a key)
- HINTS: Use an INT-0x16 keyboard service to ‘peek’ into the keyboard-queue, and omit the ‘\n’ (newline) control-code from the ‘report’ message-string