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PC Timer

tangkh asked
Medium Priority
Last Modified: 2010-04-12
I am looking into the possibility of using PC timer for the real time measurement. I know that PC always has at least 2~3 timers. I know that Timer 2 has address of 0x42,0x43 through some articles. Can anyone tell me the address of other timers? I have been searching through all possible internet sources, yet could find the detailed descriptions.
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:-???? 2-3 timers ? AFAIK, there is only one timer chip, that do
have several _channels_. One of them is PC speaker.

The original PC design has one 8253 type timer IC mapped to I/O addresses 040-043H. There are three timer sections, T0 is used for IRQ0 time tick interrupts and provides the 18.2 ticks/second main clock. T1 is used for DMA refresh and T2 is used for the speaker control. Thus none are available for use by programs unless you can use them "as is".

There are 3rd party hardware vendors that make such things. Try:



Can you tell me the registers map and the address of this timer?


I am sorry I dont mean to reject your answer. I am new to this newsgroup. When I saw "open this question to other experts", I thought I can get response from others.

Ok, no problemo. That IC is not available for program use as it is fully occupied keeping your system running.

BTW the data was from the original "IBM PC Technical Reference Manual" p. D-9 (logic diagrams) and p. 1-10 I/O Address Map.

The data sheet I have handy for the 8253 is a summary. It just shows pinout and block diagram. No registers or such. I have written code to use on *external* 8253's. Each timer section has two input lines and one output. The two inputs are Clock and Gate and the output is just that, output. The timer can be set to a variety of modes of operation including one-shot, divide by N, and simple counter. Each counter is 16-bits and can count in either BDC or binary. The original part was limited to 2MHz operation but later versions clocked at higher rates.

The 8253 is currently obsolete and has been replaced by the 8254. The 8254 datasheet is available at:


This gives you full electrical and programming specs on that device.

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Hahahaha, Ok you got the point, although my question wasn't really answered :)


On top of this, could you give me the source of the BIOS map?

Your question *was* answered, I can't help it if the answer isn't the one you wanted! (Grin). BIOS map is from same book, "IBM PC Technical Reference". This is the granddaddy reference manual of them all. It contained not only detailed technical discussions about the features and specifications of the PC but it also included complete SCHEMATICS and the source code to the ROM BIOS! If it's incompatible with *THIS BOOK* then it ain't a PC!
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