My Retro coding journey

I'm a newbie when it comes to retrocoding in C for DOS, and it's been quite a ride so far.

It took me two weeks to debug a problem with my font-loading function, and it turned out I simply wasn't allocating enough memory. The culprit? My DPMI allocation function. I need to convert the memory size to paragraphs (1 paragraph = 16 bytes), but I accidentally divided by 16 twice. Facepalm.

To be fair, the two weeks is partly because I don't do DOS development full-time. That wouldn't pay my bills.

Ever heard of DPMI? Wondering why I needed my own memory allocation function? Let's dig in.

DPMI stands for DOS Protected Mode Interface. It allows you to write 32-bit applications for DOS while still accessing BIOS functions. Pretty wild, huh? BIOS (Basic Input/Output System) is the low-level system that helps your machine boot up. On modern systems, BIOS has been mostly replaced by UEFI, but back in the day, it was the gatekeeper for hardware control.

Here's the deal: all the BIOS stuff lives in the lower memory block, the first 640KB of RAM. My C program, which is a 32-bit application, is loaded into the upper memory block. But there's a catch: BIOS is still 16-bit. If I want it to read or write to memory, it can only access that lower chunk of RAM.

This is where DPMI steps in. Normal malloc() from C won't cut it here—I need to allocate memory in the lower block and create a descriptor. Think of it as a bridge that lets my 32-bit code talk to 16-bit BIOS routines.

// Data structure holding my DOS lower memory block
typedef struct dos_block_s {
    uint16_t segment;   // Real mode segment for ES
    uint16_t selector;  // Protected mode selector for direct access
} dos_block_t;

#if defined __DOS__ && defined __386__

dos_block_t dpmi_alloc_dos_block(uint32_t size)
{
    dos_block_t dblk = {0, 0};
    union REGS regs;

    regs.x.eax = 0x0100; // DPMI function: Allocate DOS memory block
    regs.x.ebx = (size + 15) >> 4; // convert to paragraphs (1 paragraph = 16 bytes)

    int386(0x31, &regs, &regs);

    if (regs.x.cflag)
        return dblk; // Failure, return {0, 0}

    dblk.segment = regs.w.ax;
    dblk.selector = regs.w.dx;
    return dblk;
}
#else
// Stub for platforms other than dos, 
// basically to make VSCode happy.
#define dpmi_alloc_dos_block(size)
#endif

For fonts! I wanted to use a custom font for my text-mode application. I designed a sans-serif 8x8 raster font, which really changes the look and feel of the classic DOS screen.

The cool part: text-mode graphics and raster fonts are still a huge deal in the indie gamedev scene. There's something incredibly satisfying about pushing chunky pixels directly and seeing the result instantly on the screen.

To feature a decent example, check out Monogram and DungeonMode by datagoblin.

Beautiful 8x8 raster fonts that makes you want to start building a roguelike RPG from scratch.

The big picture is to build a basic music application in textmode that runs everywhere, using the old OPL2 chip that was used in Ad Lib and soundblaster cards.

Why? Because my elder brother built a music application in the 90s in Pascal. It had MIDI support and everything!

He has always been a role model to me.

Why DOS? It's easier to start with DOS for me. It's a limiting choice. But this way, I won't feel overwhelmed by the drive to build it for Windows, Linux, Mac and the Web simultaneously. Maybe just to get a foot in the door about non-web UI development. Or, to keep it short: for fun.

The code for it is still in an early stage. I'm putting together puzzle pieces here:

• https://github.com/learosema/muzimake/
• The special font loading experiment I talked about: https://github.com/learosema/muzimake/blob/main/tests/test_fnt.cpp

This journey into DPMI and custom fonts has been a massive learning experience. It might not be the smoothest path, but debugging this low-level stuff really makes you appreciate the magic of modern OS memory handling.

Next on my list? Building a proper text-mode UI for my application. Let's see how deep this rabbit hole goes...