ali-mosavian / unreal_mode_demo

A simple demo program that enables flat 4GB mem access in real mode and draws trendy gradient using the linear frame buffer (usually located above the 2GB mark) of the graphics card

What's this?

A demonstration on how to set the CPU in "un-real mode" and access the linear frame buffer of the video card (requires VBE 2.0 or higher compatibility) to draw a gradient while still being in real-mode.

Why

Short answer
Just for fun, by an old DOS programmer.

Long answer
To anyone who's unfamiliar with PC history, accessing 4GB of flat memory in real-mode doesn't sound like a big deal. So here follows a short history lesson of struggles of legacy PC programming, Up until the mid 90s DOS was quite common still. Real-mode DOS ran in a horrendous memory model where data and code were addressed using a segment and an offset. Within each segment you could address 64KB of data using the offset 0000:ffffh, however, the segments were also overlapping every 16 bytes (paragraph). As illustrated by the figure below.

[Courtesy of CIS-77 course at Bristol Community College ]

Using this segmented model you could in total access 1MB (out of which you could get to use ~620KB at best) of memory in a very convoluted way. To access any more than that you had to use hardware that could bank its memory in the sub 1MB area (i.e VESA Bios Extensions)... or by using and Expanded Memory Emulator (EMS).

Those were the struggles faced by DOS programmers. On a 386 or better, it was/is even possible to use a 32 bit register as offset as demonstrated below.

    push    word 0a000h
    pop     es
    xor     edi, edi

.loop:
    mov     al, es:edi
    inc     edi
    cmp     edi, 0ffffh
    jl      .loop

As long as your offset is within the range 0000:ffffh, anything above that will cause general protection fault. Un-real mode/big real mode hacks the CPU to let us use any offset between 0-4GB without having to run actual 32-bit code in protected mode. Thereby letting us access the VBE linear frame buffer (usually > 2GB+) and everything else that was never possible in real mode DOS.

Any real-mode DOS programs/tools will (probably) still work without any issues. Such as QuickBasic, Turbo Pascal and what not.

Caveats

• Won't work under V86 mode. That includes
  • If an EMS emulator is loaded
  • Running it directly from Windows. Although, 64-bit windows doesn't support Virtual DOS Machine (VDM) anyways...
• Instructions such as rep movs/stos only use 16 bit offsets in real mode.

How to run

For convenience gradient.asm is written in MASM syntax but is easy to port to NASM/FASM.

To compile (using MASM 6.11 or higher)

ml gradient.asm

The most convenient way to run is likely through DOSBox. To launch in the DOS command prompt, type

gradient.com

You should see the following output