How to use all memory on an IBM PC with 8086

Question

I'm developing software for the IBM PC with an 8086 processor. I want my program to use all available memory.

I know that I can use DOS int 21h function AH=48h to allocate all available conventional memory between 0 and 640K. But how do I allocate and use all RAM between 640K and 1024K?

Maybe there is not much RAM there, only video memory, which can be very small (less than 68K in total). Nevertheless, I want to use everything available.

As a historical curiosity, were there any IBM PC 8086 systems with more than 640K of RAM in use?

Answer 1

Assuming there is usable memory between 640K and 1024K (which would be rather unusual in an 8086 PC), I would recommend using DOS 5.0 or later and adding UMBs to the memory allocation strategy (there are two steps involved: add UMBs to the memory chain with 0x5803, then set the allocation strategy to include them with 0x5801). You would combine this with separate drivers to add UMBs — e.g. UMBHERC.SYS to repurpose a HGC’s video buffer, QRAM.SYS combined with a hardware EMS card, or even UMB.SYS if you have a memory board capable of backfilling RAM between 640K and 1024K.

A blunter strategy would be to try changing values in upper memory and seeing what sticks, but that would be far more fragile.

There are many discussions of adding upper memory to 8086 PCs on forums, see for example this thread using a Tandy 1000.

Another possibility is to add memory mapped between 0xA0000 and 0xB7FFF to the conventional memory allocation chains, either using memory expansion cards mapped to those addresses, or using video buffers if a compatible graphics card is installed and the corresponding video modes are never used (e.g. you have a VGA and only run text-mode programs). DR DOS’s MEMMAX can do this, as can many other memory management utilities.

As a historical curiosity, were there any IBM PC 8086 systems with more than 640K of RAM in use?

Yes, there have been quite a few. Any 8086 PC with an EMS board would qualify. In fact using an EMS board would be a much safer and time-appropriate solution to RAM problems on an 8086 PC.

(There have been many 8086 DOS systems with more than 640K of RAM directly accessible to DOS, as “conventional” memory in a contiguous block; see Who set the 640K limit?)

Answer 2

The short answer is, there is no usable RAM there so you can't use any. If you have an MDA video card, it has only 4096 bytes of video memory, and for 80x25 screen, it uses 4000 bytes already, so there is 96 bytes that are not visible. If you need more memory then you need to add memory, either just plain memory cards that can provide memory, or an EMS card which can add paged memory.

The 8086 can only address 1024 kilobytes, and when making the 5150 PC, IBM decided to split that so that up to 640 kB of RAM is possible and the rest of the memory, 384 kB is reserved for system use and expansion cards.

Some non-IBM systems could allow to have more memory and start the expansion memory area later, but if they were compatible with PC graphics there could only be up to 64k or 96k more before the start of video memory area.

Typically there is only up to 640 kilobytes, either all of it on the motherboard or in some cases some of it in the expansion slots. You can't even assume you have the maximum amount installed.

So, assuming there is a video card installed, you can use it for reading and writing, but it would be slower to read and write than main memory and everything you store there will be visible on screen, as that is the purpose of video memory. Only later cards such a Hercules, EGA, and VGA had more memory than was needed to show one page.

So if there are no special RAM cards that add general memory to the 384k reserved area, then there is no memory.

Typically, you would not be able to use the video memory in any meaningful way to store arbitrary data, and that memory would typically not be visible to DOS, not without tricks.