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0 pointsVogonPoetry5mo ago0 comments

VM in this usage means Virtual Memory - i.e. with page tables enabled. Two "processes" can use the same memory addresses and they will point to different physical memory. In Real Address mode every program has to use different memory addresses. The VM86 mode lets you to have several Real Mode programs running, but using Virtual Memory.

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oso2k5mo ago

VM does not mean Virtual Memory in this context. VM does mean Virtual Machine. When an OS/DPMI Server/Supervisor/Monitor provides an OS or program a virtual interface to HW interrupts, IO ports, SW interrupts, we say that OS or program is being executed in a Virtual Machine.

For things like Windows 3.x, 9x, OS/2, CWSDPMI, DOS/4G (DPMI & VCPI), Paging & Virtual Memory was an optional feature. In fact, CWSDPMI/djgpp programs had flags (using `CWSDPR0` or `CWSDPMI -s-` or programmatic calls) to disable Paging & Virtual Memory. Also, djgpp’s first DPMI server (a DOS extender called `go32`) didn’t support Virtual Memory either but could sub-execute Real Mode DOS programs in VM86 mode.

http://www.delorie.com/djgpp/v2faq/faq15_2.html

VogonPoetryOP5mo ago

I agree that my comment about VM was imprecise and inaccurate.

I do dispute your assertion that virtual memory was "disabled". It isn't possible to use V86 mode (what the Intel Docs called it) without having a TSS, GDT, LDT and IDT set up. Being in protected mode is required. Mappings of virtual to real memory have to be present. Switching in and out of V86 mode happens from protected mode. Something has to manage the mappings or have set it up.

Intel's use of "virtual" for V86 mode was cursory - it could fail to work for actual 8086 code. This impacted Digital Research. And I admit my experiences are mostly from that side of the OS isle.

I did go back and re-read some of [0] to refresh some of my memory bitrot.

[0] https://www.ardent-tool.com/CPU/docs/Intel/386/manuals/23098...

trollbridge5mo ago

Slight nitpick: you could run fire up V86 mode without any LDT entries.

It's also possible to run Virtual 8086 mode without paging enabled, and when in Virtual 8086 mode, the 386 doesn't care about what's in the LDT/GDT (unless an interrupt happens). In practice this is never done because the Virtual 8086 task would only be able to use the bottom 1MB of physical memory.

trollbridge5mo ago

Slight nitpick: OS/2 2.x+ did not have a way to disable paging, although you could disable virtual memory in any version of OS/2 by simply setting MEMMAN=NOSWAP.

On Windows 3.x, paging and swapping was optional - if you started it in 286 ("standard") mode. On Windows 95, paging is not optional, and it's not optional in Windows 3.11 for Workgroups either.

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oso2k5mo ago

http://www.delorie.com/djgpp/v2faq/faq15_2.html

VogonPoetryOP5mo ago

I agree that my comment about VM was imprecise and inaccurate.

Intel's use of "virtual" for V86 mode was cursory - it could fail to work for actual 8086 code. This impacted Digital Research. And I admit my experiences are mostly from that side of the OS isle.

I did go back and re-read some of [0] to refresh some of my memory bitrot.

[0] https://www.ardent-tool.com/CPU/docs/Intel/386/manuals/23098...

trollbridge5mo ago

Slight nitpick: you could run fire up V86 mode without any LDT entries.

trollbridge5mo ago

Slight nitpick: OS/2 2.x+ did not have a way to disable paging, although you could disable virtual memory in any version of OS/2 by simply setting MEMMAN=NOSWAP.

On Windows 3.x, paging and swapping was optional - if you started it in 286 ("standard") mode. On Windows 95, paging is not optional, and it's not optional in Windows 3.11 for Workgroups either.

j / k navigate · click thread line to collapse