Two CPUs

Looking through some old computer magazines from the 1980s, I was surprised at the number of advertisements for dual-CPU boards.

We don't see dual-CPU configurations now. I'm not talking about dual cores (or multiple cores) but dual CPUs. Two actual, and different, CPUs. In the 1980s, the common mix was a Z-80 and an 8086 on the same board. Sometimes it was an 8085 and an 8086.

Dual-CPU boards were popular as the industry transitioned from 8-bit processors (8080, Z-80, 6502, and 6800) to 16-bit processors (8088 and 8086, mainly). A dual-CPU configuration allowed one to test the new CPU while still keeping the old software (and data) available.

Dual-CPU configurations did not use two CPUs at the same time. They allowed for one or the other CPU to be active, to run an 8-bit operating system or a 16-bit operating system, much like today's dual-boot configuration for multiple operating systems on a single PC.

Computers at the time were more expensive and more modular than they are today. Today's computers have a motherboard with CPU (often with integrated graphics), slots for memory, slots for GPU, SATA ports for disks, and a collection of ports (video, USB, ethernet, sound, and sometimes PS/2 keyboard and mouse). All of those items are part of the motherboard.

In contrast, computers in the 1980s (especially those not IBM-compatible) used a simple backplane with a buss and separate cards for CPU, memory, floppy disk interface, hard disk interface, serial and parallel ports, real-time clock, and video display. It was much easier to replace the CPU and keep the rest of the computer.

So why don't we see dual-CPU configurations today? Why don't we see (for example) a PC with an Intel processor and an ARM processor?

There are a number of factors.

One is the cost of hardware. Computers today are much less expensive than computers in the 1980s, especially after accounting for inflation. Today, one can get a basic laptop for $1000, and a nice one for $2000. In the 1980s, $1000 would get a disk subsystem (interface card and drives) but the cost of the entire computer was upwards of $2000 (in 1980 dollars).

Another factor is the connection between hardware and operating system. Today, operating systems are tightly bound to hardware. A few people use grub or BootCamp to run different operating systems on the same computer, but they are few. For most people, the hardware and the operating system are a set, both coming in the same box.

Booting from a floppy disk (instead of an internal fixed disk) puts a degree of separation between hardware and operating system. One can easily insert a different disk to boot a different operating system.

Computers today are small. Laptops and micro-size desktops are convenient to plunk down almost anywhere. It is easy enough to find space for a second computer. That was not the case in the 1980s, when the CPU was the size of today's large tower unit, and one needed a large terminal with CRT and keyboard. Space for a second computer was a luxury.

Finally, today's processors are sophisticated, with high integration with the other electronics on the motherboard. The processors of the 1980s were quite isolationist in their approach to other circuitry, and it was (relatively) easy to design the circuits to allow for two CPUs and to enable one and not the other. The connections for today's Intel processors and today's ARM processors are much more sophisticated, and the differences between Intel and ARM are pronounced. A two-CPU system today requires much more than the simple enable/disable circuits of the past.

All of those factors (expense of hardware, ease at replacing a single-CPU card with a dual-CPU card, limited space, the ease of changing operating systems, and the ability to link two unlike CPUs on a single card) meant that the dual-CPU configuration was the better choice, in the 1980s. But all of those factors have changed, so in 2022 the better choice is to use two different computers.

I suppose one could leverage the GPU slot of a PC, and install a special card with an ARM processor, but I'm not sure that the card in the GPU slot can operate as a buss master and control the ports and memory of the motherboard. But such a board would have limited appeal, and probably not a viable product.

I think we won't see a dual-CPU system. We won't see a PC with an Intel processor and an ARM processor, with boot options to use one or the other. But maybe we don't need dual-CPU PCs. We can still explore new processors (by purchasing relatively inexpensive second PCs) and we can transition from older CPUs to newer ones (by purchasing relatively inexpensive replacement PCs).