Modularity and cross-language projects: a nostalgic look

Before I was paid to work on code for a living, it was my hobby. My favorite project from when I was young, before The Internet came to the masses, was a support library for my other programs (little widgets, games, and utilities for myself): a loadable graphics driver system for VGA and SVGA cards, *just* before Windows became popular and provided all this infrastructure for you. ;)

The basics

I used a combination of Pascal, C, and assembly language to create host programs (mainly in Pascal) and loadable modules (linked together from C and asm code). I used C for higher-level parts of the drivers like drawing lines and circles, because I could express the code more easily than in asm yet I could still create a tiny linkable bit of code that was self-sufficient and didn’t need a runtime library.

High performance loop optimizations and BIOS calls were done in assembly language, directly invoking processor features like interrupt calls and manually unrolling and optimizing tight loops for blits, fills, and horizontal lines.

Driver model

A driver would be compiled with C’s “tiny” memory model and the C and asm code linked together into a DOS “.com” executable, which was the simplest executable format devisable — it’s simply loaded into memory at the start of a 64-KiB “segment”, with a little space at the top for command line args. Your code could safely assume the pointer value of the start of the executable within that segment, so you could use absolute pointers for branches and local memory storage.

I kept the same model, but loaded it within the host program’s memory and added one more convention: an address table at the start of the driver, pointing to the start of the various standard functions, which was a list roughly like this:

  • set mode
  • clear screen
  • set palette
  • set pixel
  • get pixel
  • draw horizontal line
  • draw vertical line
  • draw arbitrary line
  • draw circle
  • blit/copy


IIRC, a driver could choose to implement only a few base functions like set mode & set/get pixel and the rest would be emulated in generic C or Pascal code that might be slower than an optimized version.

The main custom optimizations (rather than generic “make code go fast”) were around horizontal lines & fills, where you could sometimes make use of a feature of the graphics card — for instance in the “Mode X” variants of VGA’s 256-color mode used by many games of the era, the “planar” memory mode of the VGA could be invoked to write four same-color pixels simultaneously in a horiz line or solid box. You only had to go pixel-by-pixel at the left and right edges if they didn’t end on a 4-pixel boundary!

SVGA stuff sometimes also had special abilities you could invoke, though I’m not sure how far I ever got on that. (Mostly I remember using the VESA mode-setting and doing some generic fiddling at 640×480, 800×600, and maybe even the exotic promise of 1024×768!)

High-level GUI

I built a minimal high-level Pascal GUI on top of this driver which could do some very simple window & widget drawing & respond to mouse and keyboard events, using the low-level graphics driver to pick a suitable 256-color mode and draw stuff. If it’s the same project I’m thinking of, my dad actually paid me a token amount as a “subcontractor” to use my GUI library in a small program for a side consulting gig.

So that’s the story of my first paying job as a programmer! :)

Even more nostalgia

None of this was new or groundbreaking when I did it; most of it would’ve been old hat to anyone working in the graphics & GUI program library industries I’m sure! But it was really exciting to me to work out how the pieces went together with the tools available to me at the time, with only a room full of Byte! Magazine and Dr Dobb’s Journal to connect me to the outside world of programming.

I’m really glad that kids (and adults!) learning programming today have access to more people and more resources, but I worry they’re also flooded with a world of “everything’s already been done, so why do anything from scratch?” Well it’s fun to bake a cake from scratch too, even if you don’t *have* to because you can just buy a whole cake or a cake mix!

The loadable drivers and asymmetric use of programming languages to target specific areas of work are *useful*. The old Portland Pattern Repository Wiki called it “alternating hard and soft layers”. 8-bit programmers called it “doing awesome stuff with machine language embedded in my BASIC programs”. Embedded machine code in BASIC programs you typed in from magazines? That was how I lived in the late 1980s / early 1990s my folks!


I *might* still have the source code for some of this on an old backup CD-ROM. If I find it I’ll stick this stuff up on GitHub for the amusement of my fellow programmers. :)