View
230
Download
2
Category
Preview:
Citation preview
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– 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?
main memory
Interrupt Vector Table(for real-mode)
ROM-BIOS DATA AREA
tick_count0040:006C
0x00000
0x00400
0x00500
Number of timer-tick interrupts so far today (longword at 0x0046C)
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 zeromov %ax, %fs # using FS registermov %fs:0x046C, %eax # copy tick-count to EAXmov %eax, total_ticks # save in a local variable
segment-override prefix (segment used would be %ds)
Converting ‘ticks’ to seconds
total_seconds_today = total_ticks_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 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
• 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’
Channel 0
Channel 1
Channel 2
8254 PIT
8284PCLK
+5 V
CLK0
CLK1
CLK2
GATE0
GATE1
GATE2
OUT0
OUT1
OUT2
Interrupt IRQ0
DRAM refresh
speaker
Port 0x61, bit #0
Port 0x61, bit #1
AND
Port 0x61, bit #5
Port 0x61, bit #4
1193182 Hz
Counter decrements when pulsed
OUT
CLK
GATE
LSB
STATUS
MSB
LSBMSB
LATCH REGISTER
COUNT REGISTER
TIMER/COUNTER CHANNEL
8254 Command-Port
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
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
7 6 5 4 3 2 1 0
CHANNEL OUTPUT MODECOMMANDbinary/ BCD
Commands are sent to the 8254 via io/port 0x43
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
• 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…
• 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
EAX
reg (or mem)
EDX EAX
64-bit product
multiplicand (32-bits)
multiplier (32-bits)
Before executing the MUL instruction…
32-bit operands
After executing the MUL instruction…
product (64-bits)
mull reg_or_mem Here’s the instruction…
How ‘DIV’ works
EDX EAX
64-bit dividend
Before executing the DIV instruction…
dividend (64-bits)
reg (or mem) divisor (32-bits)
32-bit operand
divl reg_or_mem Here’s the instruction…
EDX EAX
32-bit remainder
After executing the DIV instruction…
two results (32-bits)
32-bit quotient
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, %eaxmov $FREQ_DIVISOR, %ecxmul %ecxmov $PULSES_PER_SEC, %ecxdiv %ecxmov %eax, total_seconds
# Now integer-quotient is in EAX, and integer-remainder is in EDX
‘Time-Of-Day’ Format
HH:MM:SS am/pm
hours
minutes
seconds morning or afternoon
So we need to compute four numerical values from the ‘total_seconds’ integer
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
• 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?
• 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…
• 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 remaindersub %ecx, %edx # computes: 2*quotient – divisor # now carry-flag is clear in case 2*quotient >= divisorcmc # complement the carry-flag bit# now carry-flag is set in case 2*quotient >= divisoradc $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
• 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
Recommended