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why did 70/80/90 only have 1 cpu if cpus where so slow?

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so my question is why did 70/80/90 only have 1 cpu if cpus where so slow?if i understand most computer only had 1 single main cpu.
for example why it was possible use for example multiple Intel 4004
instead single main cpu to make faster computers.
if money was not issue,what would prevent 4 same cpus running in single system?

Money was definitely an issue!
Technology was very primitive, slow buses, limited complexity...
Multicore systems existed, but only on very expensive mainframes.


--- Quote from: aqarwaen on March 25, 2023, 09:47:35 pm ---if money was not issue,what would prevent 4 same cpus running in single system?

--- End quote ---

As pointed out already, money absolutely was an issue. You could buy multiprocessor machines but they were expensive. (the CM-1 was commercially available in the mid-80s with up to 64k individual processors... but it wasn't exactly competing in the same market as a 4004  :-DD)

Also, you can't just throw two cpus on the same memory bus and let them duke it out. They need to be designed to share a bus, or need to have separate memory spaces and some sort of communications channel. Either way adds hardware and software complexity and cost.

Back in the 90s, I was diving deeper into computing, and was in '99 proud owner of a Tyan Tomcat 4 with dual Pentium 233MMX and a whopping 128 MB RAM.
Was able to run only under Windows NT and later FreeBSD- where I had to compile myself the SMP kernel.

The SMP stuff was mostly the issue: It required some additional hardware, the hardware designers really had to be careful about timings of the various components, and then came the big issue: The software.
Most operating systems were not suited for SMP, and those that were, imposed quite a overhead over the operations there. Like having global locks for interrupts from the network card etc.
So usually the 2nd CPU gave about 50% performance increase, and 4-way boards in servers were only deemed fit for database operations, where massive parallelization of queries in big databases could be made.
Also the user software had to be fit for SMP to have some real performance boost, otherwise one would only have the advantage of being able to run a few more singlethreaded programs on the box simultaneously.

Using more than 1 CPU needs extra effort. This is extra hardware to transfer data and also extra support from the OS or compiler to actually use more than 1 CPU. So the return for more than 1 CPU is not that great. AFAIR Linux only started to support more than 1 CPU core from around 2000 on and initially the support was somewhat limited. So much software could not use a 2nd CPU, even if present.
Even today 2 CPUs does not mean 2 x the speed.

There were a few systems with more than one CPU. A have an old CPU board with 2 x MOS6502A from the early 1980s / late 70s  (from a Comodore external dual floppy like 4040). So there were a few cases using more than 1 CPU, but that was more like the exception for a special task. AFAIK it is a somewhat crazy system with 2 CPUs capable of 2 MHz run at 1.5 MHz interleaving the access to a common memory (at least the RAM, the ROM part could be separate).
Other computers had 2 different CPUs like 6502 and Z80 to run different software, but usually not at the same time.


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