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When I run PC-DOS on my 4 core AMD Phenom chip, does it take advantage of the extra parallel CPU's? If not, is there a way to coax DOS to use all available CPU's or does this require specific developer programming at the assembly or C compilation time?

No, DOS won't use any additional CPU (*1) ever.

(Though it might run faster due them new CPUs being faster)

Quite the same way as DOS doesn't take advantage of the extended memory or additional instructions.

DOS is a

• Single CPU


• Single User


• Single Task


• Single Program


• Real Mode


• 8086

operating system.

Even through it got a few extensions over time to tap a bit into newer developments, like

• A20 Handler for HMA usage


• Utilities for extended memory usage like HIMEM.SYS or EMM386
  


• Usage of certain 286 instructions

it never left its original setup. That's what Unix, OS/2 and Windows NT were meant for.

Similar goes for third party extensions like background utilities (SideKick) or process swappers/switchers (DoubleDos et.al.), or the eventually furthest reaching combination of all, Windows (up to 98). They all added layers to manage DOS for some increased functionality, but didn't (and couldn't) change its basic workings.

Of course, application programs can use additional features of a machine, no matter if it's new CPU instructions, a new graphics card, more memory, or in this case an additional CPU. Much like DOS-extenders allowed protected mode programs to run as DOS applications, most notably maybe DOS/4GW, which got huge popularity due being delivered with Watcom compilers.

Then again, I do not know of any extender allowing the use of concurrent CPUs.

It's worth noting that none of these additions - including the application specific DOS-extenders - were changing the basic paradigm of DOS as a single-user, single-task operating system.

*1 - What to call it? CPU, core, hyperthread, socket? Well, in ye good olde times of simple microprocessors we couldn't care less. Nowadays it gets blurry. With multiple processing units of various quality it gets hard to qualify. For most purposes the professional world continues to call the device (IC) a CPU, but avoids the term when it comes to system building and talks now about sockets, leaving open how many processing units the IC plugged in will have - or if at all. Often the term core is used instead, but thanks to Intel using this as well as a brand name, it got blurred again. So for all RC.SE purposes I stay with the term CPU, as vague as it is, and qualify if needed.

If by IBM DOS you mean IBM PC DOS, which was a rebrand/derivate of MS-DOS, then the answer is no - DOS will only ever support a single core. HyperThreading and multiple cores is simply not supported by DOS.

Making DOS use multiple cores would be a major operation. Firstly DOS would have to support multitasking. It could not be task switching or cooperative multitasking, it would have to be full pre-emptive multitasking. Then it would need to be developed to support SMP (symmetric multiprocessing). Both of these tasks would be major undertakings - their peers in the 90s took years to get reliable and efficient multiprocessing support.

It is possible that a single application could take advantage of multiple cores if it used DOS as a bootstrap and then kicked DOS out, and essentially became its own operating system.

If you're looking for a 90s-era operating system that supports multiple cores, then you would be looking at Linux (2.0 or later), Windows NT 4 or OS/2 Warp.

DOS itself won't do anything to boot up the "extra" cores in a multicore system, running only on the boot CPU.

A program that does that is normally called an operating system. You could certainly have a program that takes over from DOS. Maybe even one that saves the previous DOS state and could exit back to DOS. But nobody's written such a thing. You can probably find bootloaders that will load Linux from DOS, though. It would involve taking over the interrupt table and so on.

https://stackoverflow.com/questions/980999/what-does-multicore-assembly-language-look-like includes an answer with some details on what you'd need to do on x86 to bring up other cores to send inter-processor interrupts (IPIs) that bring up other cores.

CPUs with fewer cores tend to have higher guaranteed clock speeds, because each core has a higher power budget. But CPUs that support "turbo" typically have a high single-core turbo clock they can use when other cores are idle.

Modern Intel CPUs with more cores also tend to have more L3 cache; a single core can benefit from all the L3 cache on the whole die. (Not on other CPUs in a multi-socket system, though).

If you're using a DOS extender that allows a DOS program to access more than 1MiB of RAM (e.g. running in 32-bit protected mode), then it might actually benefit from having more than the 3MiB of L3 cache that a low-end dual-core system has.

But otherwise DOS can't use more memory than the smallest L3 caches on modern mainstream x86 CPUs. (Or on Skylake-X, the per-core L2 caches are 1MiB, up from 256kiB, so other than unacheable I/O / device / VGA memory, everything would be a cache hit with 13 cycle latency, much faster than ~45 cycle latency L3 in a dual or quad core!)

But there's a downside to having more cores on Intel CPUs: L3 cache latency. They put a slice of L3 along with each core, and cores + L3 are connected by a ring bus. So more cores means more hops on the ring bus on average to get to the right slice. (This is reportedly even worse on Skylake-X, where a mesh connects cores. It's odd because a mesh should mean fewer hops.)

This extra latency also affects DRAM access, so single-core memory bandwidth is better on dual / quad core desktop CPUs than on big many-core Xeons. Why is Skylake so much better than Broadwell-E for single-threaded memory throughput?. Even though the Xeon has quad-channel or 6-channel memory controllers, a single core can't saturate them and actually has worse bandwidth than a single core of the same clock speed on a quad-core part. Bandwidth is limited by max_concurrency / latency.

(Of course this doesn't apply to L2 cache hits, and 256kiB L2 is a good fraction of the total memory that DOS programs can use without a DOS extender. And 2, 4, or 8 MiB of L3 cache is pretty huge by DOS standards.)

AMD Ryzen is different: it uses "core clusters" which each group of 4 cores share an L3 cache. More total cores won't give you more L3 that a single core can benefit from. But within a cluster, L3 latency is fixed and pretty good.

The answers above are correct about the core usage, but it would still be faster than old machines because the clock (MHz/GHz) of the CPU is faster. This is actually problematic on many of the old games because things happen faster than you can react.

If you wanted to test/play without committing to a wipe/load scenario, you could always try FreeDOS on an emulator or on an old hard drive.

When PC starts, it launches only one CPU core (so called bootstrap CPU). OS is executed on this CPU. Then, OS starts other cores by sending IPI (inter processor interrupt). To do this, OS first switches PIC (interrupt controller) to APIC (advanced) mode and uses its registers.

DOS is not aware of other CPUs and APIC, and does not know how to send IPI.

So, DOS can't even start other cores. They simply turned off and do not work.

If you want to use them, you would have to deal with APIC by yourself. It is not easy task, though. See https://wiki.osdev.org/Symmetric_Multiprocessing