Orsdxy

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 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 old 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.

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.