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Using the 8254 Timer-Counter. Understanding the role of the system’s 8254 programmable Interval-Timer/Counter. Displaying ‘Time-Of-Day’. Algorithm steps: Get the count of timer-interrupts so far today Convert these ‘timer-ticks’ into seconds

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using the 8254 timer counter

Using the 8254 Timer-Counter

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

displaying time of day
Displaying ‘Time-Of-Day’
  • 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
where s the tick counter
Where’s the ‘tick’ counter?

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

getting the tick count
Getting the ‘tick’ count
  • 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)

converting ticks to seconds
Converting ‘ticks’ to seconds

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 pit
The 8254 PIT
  • 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
input output frequencies
Input/Output frequencies
  • 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
three timer counter channels
Three timer/counter ‘channels’

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

counter decrements when pulsed
Counter decrements when pulsed

COUNT REGISTER

CLK

MSB

LSB

OUT

MSB

LSB

LATCH REGISTER

GATE

STATUS

TIMER/COUNTER CHANNEL

8254 command port
8254 Command-Port

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

programming a pit channel
Programming a PIT channel
  • 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
standard bios programming
Standard BIOS programming
  • 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)

consequently
Consequently…
  • 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)
how mul works
How ‘MUL’ works

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

how div works
How ‘DIV’ works

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

implementing the conversion
Implementing the conversion
  • So use MUL and DIV to convert ‘ticks’ into ‘seconds’, like this:

# total_seconds = ( total_ticks * FREQ_DIVISOR ) / PULSES_PER_SEC

mov total_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

time of day format
‘Time-Of-Day’ Format

HH:MM:SS am/pm

hours

seconds

morning or

afternoon

minutes

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

our four time parameters
Our four time-parameters

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;
a subtle refinement
A subtle refinement
  • 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
how to implement rounding
How to implement rounding?
  • 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;
  • But this uses a multiply and a conditional jump-instruction (inefficient!)
avoiding inefficiency
Avoiding inefficiency…
  • 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!

in class exercise
In-class exercise
  • 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
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