Electronic – Does ISA bus (or PC/XT bus) have some means of arbitration to resolve bus contention


My understanding of ISA bus is that the CPU places an address on the bus and any expansion card is free to respond to that address by taking control of the data bus. I presume that tri-state buffers are used. So my question is, are there any arbitration mechanisms in ISA and/or PC/XT-bus to prevent two cards from responding to the same address? My understanding of tri-state logic is that if this were to occur the tri-state buffers would likely get fried. Perhaps there was some sort of lesser mitigation to prevent such damage in the case of contention?

Looking online I see no mentioning of arbitration or damage mitigation, and yet I also know no stories of people with fried ISA expansion cards due to address overlaps.

Best Answer

PC and XT

The original IBM PC simply extended the Intel chipset bus to connectors using buffer drivers. The clock rate on the card bus was the exact same as the clock rate used for a CPU cycle. So with approximately \$4.77\:\text{MHz}\$ (derived by dividing by 3 a \$14.31818\:\text{MHz}\pm 5\:\text{ppm}\$ crystal rate) on the PC's CPU, this meant that a typical 6-cycle, 8-bit I/O bus transaction would take about \$1.26\:\mu\text{s}\$. This was consistent with the technology at the time, so boards could decode and latch addresses using middle-of-the-road (in terms of speed) and reasonably-priced devices. IBM would eventually published a fairly complete set of documentation on the IBM PC, XT, and PC/AT that included detailed schematics that were well laid out and understandable and a complete listing of their BIOS source code (in assembly), as well.

The PC and XT simply used the bus design that reflected Intel's chip design, without extension features (that I'm aware of.) If you tried to increase the clock rate of the CPU, then the clock rate of the bus would also increase and this put pressure on the boards. But I don't recall many attempting to do this, so it wasn't an issue.


With the advent of the PC/AT and the 80286, a new 16-bit I/O transaction and 16-bit memory transaction became available. Intel also changed over to the new 82284 clock gen chip and the 82288 bus control chip. Additional DMA channels and interrupt signal lines were added by IBM and an arbitration transaction was added so that add-on cards could replace the platform CPU as the bus owner. (A little more on that, later.)

The new standard limit for the CPU was now \$6\:\text{MHz}\$. The bus rate was similarly increased and newer boards needed to keep up. IBM also introduced a number of new cards for the system.

The 80286 had four more address lines (going from 20 to 24) and could now enter a new protected mode of operation to gain access to these new lines. While Intel was able to allow the transition from real mode operation to protected mode using appropriate software instructions, they were rushed to get the chip out to the marketplace and did not manage to successfully field the new CPU with the ability to switch back to real mode. As a result, the only way back from protected mode to real mode was through a processor reset. IBM handled this problem through the keyboard interface, using the keyboard (and memory in the calendar IC they used) to force a hardware reset when instructed to do so. The BIOS supported transitions back and forth between modes and was able to hide the fact that the keyboard needed to reset the CPU each time a request was made to get back to real mode operation.

Wider bus transfers on the PC/AT bus now also supported faster bus cycle rates; a "byte swapper" was used to port around low order and high order bytes on the bus; and the new refresh cycle logic used discrete circuits.

The rush for more CPU speed

People quickly discovered that they could increase the clock rate of their expensive IBM PC/AT to about \$8\:\text{MHz}\$ by simply replacing the clock crystal. I did this and found that I could successfully push the system and the boards I used to about \$8.5\:\text{MHz}\$ before things started to get iffy. (I couldn't reach a consistent \$9\:\text{MHz}\$ on my system, so I settled in at \$8\:\text{MHz}\$ and left it there.)

The level of skill needed (and tools required) to design a motherboard was relatively low at the time. Almost anyone could find inexpensive parts and do decent layout that would work fine at these frequencies. And a lot of "mom and pop" motherboard makers soon began to enter the scene. (IBM's price point was very high for most people.)

Perhaps the first truly successful (able to emulate the IBM hardware with 99% compatibility) PC replacement was Kaypro's 286i product. Before this, there were usually too many "issues" to make the products sufficiently acceptable to the business market (though hobbyists were often okay.) Kaypro's entry was about US$2k cheaper than IBM's, so it very quickly rolled out.

As more and more competitors solved the compatibility issues and began to compete, Intel started to roll out faster spec'd 80286 CPUs, too. Board makers would incorporate these newer CPUs, include faster logic chips so the bus could run faster, and we began to see \$8\:\text{MHz}\$, \$10\:\text{MHz}\$, and even \$12\:\text{MHz}\$ offerings. But this almost immediately put pressure on the add-on cards. Older cards simply couldn't be used and newer ones were too few, too far between, and consumers faced buying a faster system that greatly reduced the number of add-on cards they could buy and successfully use.

While a few companies attempted to isolate the add-on card bus rate from the internal Intel bus rate with discrete chips (with some success), the sheer number of "mom and pop" motherboard makers and the need to separate the clock rate of the CPU from the cycle time of the bus opened the door for a new company, Chips and Technology (aka C&T), to produce an ASIC that got this job done. Very quickly after, new motherboards entered the market allowing the ISA bus cycle time to be kept (relatively) independent of the Intel CPU clock rate. Since Intel was meanwhile continuing to increase the maximum CPU frequency, this was a godsend to the many competitors, who didn't have the internal horsepower or financing to develop ASICs but who could certainly use them in new products.

As a result, the "frequency wars" started in earnest and there was hardly a month going by where there weren't new motherboard offerings with increasing CPU clock rates. The decoupling of CPU frequency from bus frequency was a huge win for C&T, too, who did quite well in the process.

Just as a note, I believe the decoupled ISA bus operated asynchronously to the platform CPU with one exception: the RESET line to the platform CPU.

I/O and Memory and DMA

The I/O and memory bus transactions are distinct, but in most ways quite similar to each other. It's just that different boards would respond. The original 8-bit I/O transaction, for example, was 6 bus cycles long. But with the PC/AT a newer, wider bus a 3-cycle I/O was included.

It was the job of each add-on board to latch and decode the address and associated signals they were interested in (IOR or IOW, for example, for cards responding to I/O bus cycles.) They then had a certain number of clocks to respond for a standard transaction. An I/O card could, however, assert IOCHRDY if it wanted added bus cycles to complete its transaction.

With the advent now of both 8-bit and 16-bit transactions with the PC/AT ISA bus, a few issues arose. For example, a 16-bit I/O slave add-on could not force the bus master (which may or may not be the platform CPU) to execute a 16-bit access when the owner only wants an 8-bit. Similarly, a bus owner intending on a 16-bit access cannot order an 8-bit slave add-on to perform a 16-bit access. So there are added signal lines to aid in these circumstances.

The DMA access cycles were a little different from the other two in this sense: DMA would use simultaneous activation of I/O and memory command signal lines to allow data to be placed onto and retrieved from the bus during the same cycle. Here, for example, the address placed onto the bus is for memory and NOT for the I/O card (which should not use it.) (The AEN is activated to indicate to the I/O card not to use the address.)

Add-on boards responding to I/O addresses were set at unique locations to avoid conflict. IBM provided guidance about this for important cards (video display, serial port, parallel port, interrupt controller, and so on), but many add-on board makers would also include means to adjust the I/O address so that if you used two or more of their boards, they would work okay together. In general, the system worked pretty well and there were few problems. (Most issues related to the graphics memory required for various types of display controller boards.)


Technically, there actually IS an arbitration cycle. It's just not what you are asking about. Instead, it is a means by which another bus master (presumably residing on an add-on card) can claim ownership of the bus as the master. This cycle actually starts out looking like a DMA transfer cycle and it is the DMA controller which first responds. The would-be bus master then has a fixed time in which to assert MASTER and obtain ownership. The DMA controller then tri-states its own address, command, and data signals. (I worked with a team on a MIPS R2000 add-on card for the IBM PC/AT, circa 1986.)