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clh333
February 6th, 2016, 08:00 AM
I'm trying to understand how the IBM XT communicates with peripherals and adapter cards through its bus. I would appreciate the recommendation of a technical reference that explains the subject more fully than Wikipedia.

At this point I have a general or "block diagram" understanding; the XT bus has 20 address lines = 1,048,576 separate addresses; of these some are memory locations, some ports, the BIOS etc. The Intel 8080 and 8237 can communicate with these addresses. Devices keep synchronized with one another by means of the system clock and signal their need to use the bus with signal voltages on control lines.

I'd like to bring my knowledge to the level of understanding how devices communicate, why they occupy the addresses that they do and how the system differentiates between them. Obviously the wrong signal sent to the wrong address or at the wrong time could be fatal, so how is this avoided?

Thank you for your suggestions for a reference.

-CH-

gslick
February 6th, 2016, 09:50 AM
I'm trying to understand how the IBM XT communicates with peripherals and adapter cards through its bus. I would appreciate the recommendation of a technical reference that explains the subject more fully than Wikipedia.

Thank you for your suggestions for a reference.
-CH-

You probably can't go wrong with a used copy of this book for just a couple of bucks on Amazon:

http://www.amazon.com/ISA-System-Architecture-3rd-Edition/dp/0201409968
ISA System Architecture, MindShare Inc., Don Anderson, Tom Shanley

Preview of contents:
http://www.mindshare.com/files/ebooks/isa%20system%20architecture%20(3rd%20edition).pdf

Stone
February 6th, 2016, 10:34 AM
Try the IBM PC XT Technical Reference. It's even available in PDF.

sergey
February 6th, 2016, 10:54 AM
XT bus is pretty much just buffered 8088/8288 signals. Get a good book on Intel 8086/88, or find and read material from the internet. You can also look at some DIY projects that interface to ISA. Don't worry about DMA too much. Only a few devices actually use it. Yeah... and ISA or XT bus is not a synchronous bus, while CPU clock is present, data transfers are not really synchronized to it.

MarsMan2020
February 6th, 2016, 11:28 AM
I'm trying to understand how the IBM XT communicates with peripherals and adapter cards through its bus. I would appreciate the recommendation of a technical reference that explains the subject more fully than Wikipedia.

At this point I have a general or "block diagram" understanding; the XT bus has 20 address lines = 1,048,576 separate addresses; of these some are memory locations, some ports, the BIOS etc. The Intel 8080 and 8237 can communicate with these addresses. Devices keep synchronized with one another by means of the system clock and signal their need to use the bus with signal voltages on control lines.

I'd like to bring my knowledge to the level of understanding how devices communicate, why they occupy the addresses that they do and how the system differentiates between them. Obviously the wrong signal sent to the wrong address or at the wrong time could be fatal, so how is this avoided?

Thank you for your suggestions for a reference.

-CH-

The original PC and XT technical references have a lot of good information in them.

I have - http://www.amazon.com/80X86-IBM-Compatible-Computers-Interfacing/dp/013061775X/ref=sr_1_1?s=books&ie=UTF8&qid=1454790378&sr=1-1&keywords=the+80x86+ibm+pc+and+compatible+computers - which has a good overview both the hardware and assembly language programming. Also, $2 used on Amazon.

sergey
February 6th, 2016, 01:46 PM
At this point I have a general or "block diagram" understanding; the XT bus has 20 address lines = 1,048,576 separate addresses; of these some are memory locations, some ports, the BIOS etc.

IBM PC/XT has a separate (from memory) I/O space. The 8088 CPU generates 16-bit address (65536 different addresses). But XT motherboard and various extension cards tend to decode only lower 10-bit of address, so effectively the range is reduced to 1024 addresses - that is from 000h to 3FFh. Lower 512 addresses used (or reserved) for peripherals on the motherboard. So effectively extension cards can use 200h to 3FFh. There is a lot of reference material on how these ports are allocated. You can start here (https://en.wikipedia.org/wiki/Input/output_base_address).

Memory is indeed uses 20-bit address, which gives 1 MiB of memory space. Lower 640 KiB (00000h to 9FFFFh) are used by RAM (conventional memory), next there is a block which normally used by display cards (0A0000h to 0BFFFFh), next there is an area that used by extension cards either for BIOS extension ROMs, or for RAM (e.g. EMS window, or UMBs), finally last 64 KiB (0F0000h to 0FFFFFh) are allocated for BIOS and ROM BASIC.



The Intel 8080 and 8237 can communicate with these addresses. Devices keep synchronized with one another by means of the system clock and signal their need to use the bus with signal voltages on control lines.

I am sure you meant Intel 8088. And yes both CPU and DMAC can communicate with memory and I/O devices, except that DMAC can only generate 16-bit memory address, so there is a 4x4-bit page register that provides upper 4-bit of address. Also DMAC does not normally generate I/O addresses - it uses /DACK signals to select I/O devices instead. System clock is not used to synchronize the devices, and XT bus is an asynchronous bus. There are a few signals that control data transfer (/MEMR, /MEMW, /IOR, /IOW).



I'd like to bring my knowledge to the level of understanding how devices communicate, why they occupy the addresses that they do and how the system differentiates between them. Obviously the wrong signal sent to the wrong address or at the wrong time could be fatal, so how is this avoided?


On XT bus only CPU and DMAC can initiate memory or I/O transactions (as opposed to real AT ISA bus, which has some bus mastering support). Basically the CPU or DMAC puts the address it wants to read from or write to on the address bus, and in case of the write the CPU will also output the data byte on the data bus. Next one of the control signals I've mentioned above activated (that is switched to LOW level). That actually initiates the transaction.

DMA transfer is a little bit more complicated: IBM PC/XT supports only memory to I/O and I/O to memory DMA (no memory to memory). The DMAC provides the memory address, and in case of memory to I/O it activates /MEMR, one of /DACK signals, and /IOW. In case of I/O to memory it activates /IOR, /DACK, and /MEMW. So that the data flows directly between memory and I/O.

Additionally IBM PC/XT has a bus arbitration circuit, which makes sure that CPU is not accessing the bus during DMA cycles.

Your another question has to do with address decode. Each device (memory, or I/O) normally has a chip select input (/CS, or /CE), and there are address decoders (or comparators) that have their inputs connected to the address bus, and outputs to these chip select inputs. So basically a decoder always try to decode the address present on address bus, and if that address matches the device (or memory) address, it would select the chip. That doesn't initiate the transaction yet, but just select the device used for the transaction.

My answers above are oversimplified, for example they don't take timing into consideration.

Wrong signal to wrong address: I don't really understand what is "wrong signal", but there are a few possibilities:
- Non existent address being read or written - probably nothing bad will happen, except that the read value would be garbage
- Multiple devices are set to decode the same address - in case of write (to device), both of devices will get the data. In case of read - it would be bus contention. Normally it shouldn't damage anything (not enough time to do any damage), but potentially it could :-)
- Device is being written a garbage/incorrect value or command. It really depends what device we are talking about, and what is that incorrect command. The answer could vary from no damage at all to a corrupted disk, or a damaged monitor...

It looks that you don't really understand how Intel 8088 CPU works, so I'd recommend reading a college book on it, e.g. The 8086/8088 Family: Designing, Programming and Interfacing (http://www.amazon.com/8086-8088-Family-Programming-Interfacing/dp/0132467526). That should be more useful than reading a reference manual (that normally assumes some level of familiarity with CPU architecture).

1ST1
February 6th, 2016, 03:41 PM
You also can read the technical reference manual of one of the best early XT clondes, Olivetti M24 -Theory of Operation (http://bitsavers.informatik.uni-stuttgart.de/pdf/olivetti/m21_m24/Olivetti_M21_M24_Theory_of_Operation_Nov84.pdf)

It describes the function of the machine very detailed, with all TTL and GAL logics.

clh333
February 7th, 2016, 02:57 AM
Thanks to all for your suggestions. I will acquire the publications you have cited.

I used the term "reference materials" somewhat imprecisely; essentially I meant books that would explain the theory of operation and / or give practical examples. By "wrong signal to wrong address" I meant possibility three in Sergey's list.

Sergey: You're right! I don't even know what I don't know. But the overall objective is to learn enough to build something that would interface with the XT bus through an expansion port. Following published examples I've built stuff that interfaced through parallel or serial ports but never had the nerve to stick something in an expansion slot that wasn't commercially produced.

Thanks again to all who responded.

-CH-

gslick
February 10th, 2016, 02:07 PM
If you like physical books here's a couple of other reference books which you can pick up used on Amazon for basically just the cost of shipping:

http://www.amazon.com/dp/B0006EW1NO
AT bus design: Compatible with IEEE P996, 8 and 16 bit ISA, E-ISA, and EISA design
Edward Solari
978-0929392080

http://www.amazon.com/dp/0929392159
ISA and EISA Theory and Operation
Edward Solari
978-0929392158

I have a copy of the first one. Might have the second one somewhere too.

clh333
February 11th, 2016, 02:32 AM
Thanks again for your suggestions.

Yes, I like physical books. Every year I give the local library a couple of cartons of cast-offs.

I have obtained or ordered a copy of each of the suggested titles. This being Amazon, who knows when they will get here.

-CH-

clh333
February 13th, 2016, 05:04 AM
BTW, found another book, Upgrading and Repairing PCs byScott Mueller (QUE, 2004) which I wish I had read years ago. Lots of information (1500+ pages) and at $4.60 with shipping only about $0.60 per pound.

-CH-

Stone
February 13th, 2016, 05:18 AM
I have 6 different editions of that book in PDF format. :-)

T-R-A
February 13th, 2016, 05:54 AM
Upgrading and Repairing PCs byScott Mueller
-CH-

Yep, that's the definitive standard for repairing PC's. Had the hard-copies for the first several editions...like Stone, found the PDF version is much lighter :)

clh333
February 22nd, 2016, 03:03 PM
One more reference that I found and like: The Embedded PC's ISA Bus: Firmware, Gadgets and Practical Tricks, Ed Nisley, 1997 Annabooks.

-CH-