Is there a way to cut idle CPU usage under Multitasking MS-DOS 4.0?

Question

The operating system variously known as ‘Multitasking MS-DOS 4.0’ or ‘European MS-DOS’, to avoid confusion with the better-known single-tasking MS-DOS 4.0, has the mildly irritating flaw that it doesn’t invoke the HLT instruction that stops the CPU when the system is idle. This has the effect that whenever the OS is run in a virtual machine, CPU usage of the host spikes to 100%, whether the guest is actually doing anything or not, inevitably leading to my laptop CPU fan revving up with an all-too-familiar noise.

There doesn’t seem to be a simple, obvious way to remedy this. Unlike the mainstream edition of the OS, there is not an idle interrupt to which one can hook a TSR which can invoke HLT on its own. Having studied (what I assume to be) the kernel idle loop in a debugger, there doesn’t even seem to be any room to squeeze in a HLT instruction by patching the kernel. Here it is disassembled:

0506:3DE0 0E                PUSH    CS
0506:3DE1 1F                POP     DS
0506:3DE2 FB                STI
0506:3DE3 F6066105FF        TEST    BYTE PTR [0561],FF
0506:3DE8 74F8              JZ      3DE2
0506:3DEA E832FD            CALL    3B1F
0506:3DED EBF1              JMP     3DE0

Is there a way to inject a halt instruction into the idle loop somewhere?

Answer 1

In place and without changing other code: No

Simply because there is no shorter encoding of that sequence - that is unless we rely on

• additional information how (and when) the tested byte changes

and/or

• how and when values set here gets reused or not

and/or

• certain register setup prior to this routine.

---

The basic issue here is that a HLT instruction will end/continue operation after any arbitrary interrupt was handled, but it's not said that every interrupt will as well set the flag byte at CS:0561h. All that do not set that byte should of loop back and halt again.

---

There is always the usual method of jumping out by replacing the test instruction by a call to some new routine, intended to whatever is necessary - like

    HLT
    TEST xxxx
    RET  

The 5 bytes of the TEST instruction is long enough to even hold a far call, so it can be located anywhere. This method has the advantage that all other code is untouched.

---

Possible Other Ways depending on code/workings outside the core loop:

Of course if it can be guaranteed that certain data is present, some instructions can be used for patching like

• if DS is already equalling CS when entered, then the PUSH/POP pair can be dropped and their space repurposed to add a HLT instruction

0506:3DE0 90                NOP
0506:3DE1 FB                STI
0506:3DE2 F4                HLT
0506:3DE2 F6066105FF        TEST    BYTE PTR [0561],FF
0506:3DE8 74F7              JZ      3DE1
0506:3DEA E832FD            CALL    3B1F
0506:3DED EBF1              JMP     3DE0

Likewise if it's guaranteed that DS is not used later on with the assumption of still having the same value as CS, they can be replaced by a segment override prefix freeing up a much needed byte for the HLT instruction.

• if DS gets reloaded afterwards (like in the function starting at CS:3B1Fh) without being uses first, the following patch might work:

0506:3DE0 FB                STI
0506:3DE1 F4                HLT
0506:3DE2 2EF6066105FF      TEST    CS:BYTE PTR [0561],FF
0506:3DE8 74F6              JZ      3DE0
0506:3DEA E832FD            CALL    3B1F
0506:3DED EBF1              JMP     3DE0