MS-DOS Memory Management on x86

Mebibyte... Ugh.

this is a great video, subscribed

The difference between the 8086 and 8088 is that the 8086 has 16 of it's address pins duplexing as data pins, while the 8088 only has 8 of them duplexing as data pins, but they are otherwise identical. This means that the 8086 is capable of accessing it's memory 16-bits at a time instead of 8-bits like the 8088. What I'm curious about is whether or not any motherboards actually implemented this. Given that the 8086 was more popular among business-oriented machines, I'm guessing somebody did, but I'm not familiar with any specifically.

Nobody who used a computer in that time would call a megabyte "mibibyte".

From my experience the memory area above 640 Kb was only available as RAM when a memory manager program like 386MAX or QEMM was installed. DOS 5.0 and higher has a similar memory manager included. Plus you could only do this on a 386 or higher CPU. There were some DOS games that needed this because they needed 600 Kb+ of RAM to run. These memory manager programs also supported the EMS memory interface in software by reserving a 64 Kb UMB area. They also scanned the entire upper memory area to map all free UMB's which included unused areas that were reserved for the system BIOS Typically these programs would load device drivers and TSRs into these recovered UMBs.

the good old days of messing with autoexec.bat and config.sys to to free up memory to maybe run a game

You just have to know your config.sys and autoexec.bat files.

"We'll never need more than 640K " - some guy that was completely wrong.

ms tres

To get Falcon3.0 with expansion packs running.. you needed to know all this shiet in this video - front and back.

The first KB was not occupied by the BIOS. It was RAM and was the BIOS Data Area (BDA); the values were populated by the BIOS. The first KB was actually the Interrupt Vector Table (IVT), then came BDA values for items such as COM/LPT port addresses, memory size, keyboard buffer, soft reset behavior, etc. MS-DOS boot sector would load initial code to 0:7C0, and would use 70:0 as the DOD data area.

Bah humbug, I think no one needs more than 64k 😂

no, el 8088 es una variante del 8086, mas economica, el 8086 es de 1976 y el 8088 es de 1979

Talk about a blast from the past! When I was a teenager (pre-internet), I remember fighting with DOS and reading deep into manuals to squeeze out every free byte of RAM for my games. It could be quite intricate, and it took a while. It was almost like a game itself to find the right combinations.

i got specialty smartdrv, mouse, mscdex, and this special emm386(don't think it was qmm) that could use hard drive space as "ram" is you didn't have enough for a game, I used it to try to run magic carpet, on my 4mb 486 sx 25, packard bell. My computer did com with all these util Free cds. I did manage to get 621 free after all that.
Does anyone know where I might find a Dos app called no.exe
its usage would be zipping all contents of a folder with pkzip, so after zipping .
no .zip del *. So basically one could make a batch file, to add hidden attribute to the prompt but i liked the exe because it was shareware and because it told you to pay or else.
leaving you with just your zip. I would use it to compress on the fly different games when i wanted to play them and only a 250mb hard drive.

I can't find any info about no.exe

I also made a dos 62 and arachne and a ramdisk to book off single floppy with dos network support and mouse etc. I was always trying to push a limit somewhere.

What a nice stroll down memory lane, pun intended.

I* was interested to watch this movie. But when I herad the computer voice, i stop watchin after 11 sec

"What about one eighty-s-"
"Shhh, we don't talk about 80186."

this is neat. this was the explainer i needed 20 years ago. this is a summary of my decades of confusion and learning and 'eventually sorta getting it'. its great to see this all summarized in this way. it really is the top of the pyramid for an explanation of all of this history.

I remember once in 1990's I made a (test) program for bank switching with himem.sys :)

At that time my data usually fitted in the 640K RAM so I did not used bank switching for anything more than test.

What memory management?
Who in their right mind cares about DOS 6.22 in the year 2023?

Popped up recently for me too.
I don't think HIIMEM or 286 can do UMB mapping though, that needs EMM386/QEMM and the v86 capability of the 386.
I might be misremembering, but I think expended memory or LIM EMS (Lotus Intel Microsoft) was originally intended to allow bigger 123 spreadsheets
RAM on an ISA card was mapped into (original spec, a 64k page frame of 4x 16k pages) similar to the way 8 bit home computers mapped extra ROM and RAM.

EMM386.EXE twitch HIMEM.SYS flinch oh no my ptsd

I remember using this - it was a nightmare!

First time I've ever heard someone unironically say "mebibyte", but good high-level explanation of a low-level topic. High level because questions segment/offset and selector/offset weren't discussed, nor why some things require you NOT run EMM386 but might work if you had an EMS card installed, how DOS could see memory above 1MB in real mode, why load order mattered as much as memory locations for where drivers went in memory when running memmaker, etc? That sort of thing …

For how "simple" it is, MS-DOS memory management was a BIG topic. Might be too low-level for your audience though.

What does the address space look like if you did not have 1MB of Memory , specificaly if you only had 156k of memory; where does the BIOS, expansion ROM and Video?

Really enjoyed this, and learned a lot! Thank you!!

Personally I don’t mind the use of MiB and similar terminology to be clear that you’re talking about base 2. Even if it is the HDD manufacturers’ fault we live in a world where MB and KB is ambiguous now.

Isn't this mebibyte stuff just made up by HDD manufacturers? Why not just use megabytes like everyone else. No one thinks a megabyte is 1000 kilobytes!

Saying MiB instead of MB kinda makes you sound like you have a speech impediment, although I know you try to be as accurate as possible.

Windows 3.1 still ran fine in 286 mode. It would even default 286 mode if you did not have much RAM, and at least with not much memory, it would actually run faster in the 286 mode (I know that because we had a computer with 2 MiB RAM). But you did not get the full features, like virtual memory and pre-emptive multitasking of DOS applications.

Loved the video. Thanks for sharing your knowledge

Great video. This is so nostalgic. I fondly remember messing with himem.sys, TSRs etc. as a kid. At the time, I didn't really know much about real vs. protected mode though. I hope to see more like this.

It's kinda weird to see the use of the kibi- & mebi- prefixes in a DOS video since these weren't a thing then and primarily a construct by the hard drive industry given (il)legitimacy by the IEC to redefine the kilo-, mega- etc. prefixes.