rpi4-osdev/part12-wgt

Writing a "bare metal" operating system for Raspberry Pi 4 (Part 12)

Porting the WordUp Graphics Toolkit

Back in the mid-1990s (when I was young!), programmers who wanted to build their own games didn't have rich frameworks like Unity. Perhaps the closest we got was the WordUp Graphics Toolkit, which I came across on the Hot Sound & Vision CD-ROM - a BBS archive. If you have a moment, perhaps use Google to see what "bulletin board systems" were... nostaglia awaits!

Much like my very simple fb.c, the WGT provides a library of graphics routines which can be depended upon for reuse. This library, however, is much more fully-fledged than mine, and makes it easy to build sprite-based games (like Breakout, Space Invaders, Pacman etc.).

The directory structure

As I port the WGT to my OS (a.k.a. make it work on my OS), I am using the following directories:

• bin/ : for WGT binary files (fonts, sprites, bitmaps etc.)
• controller-ios/ : a sample Swift BLE controller for the iOS platform
• controller-node/ : a sample Node.js BLE controller
• include/ : now contains wgt.h and wgtspr.h too (header files necessary for WGT code)
• samples/ : sample "kernels" for my OS which exercise certain WGT library functions. To build them, copy one of these (and only one at a time) to the same directory as the Makefile.
• wgt/ : the library itself. Where possible, I have stayed true to the original code, but do bear in mind it was written for the x86 architecture and we're on AArch64!

Please note: I am neither a Node.js developer, nor a Swift developer, and so the controllers are purely samples that serve my purpose. They are not intended to be exemplars! I am very aware of the multitudinous problems with both...

Building

So... to build the first WGT sample simply type cp samples/wgt01.c . from the top-level directory, and then type make. When you boot with the generated kernel8.img you will see the screen go into 320x200 (VGA!) mode and draw a white line from corner to corner. If you do, the library is doing its stuff!

boot/boot.S changes

We're still booting into a multicore environment (just in case we need it). There are a few significant changes to boot/boot.S though. I will write more on these later, but (for now) they are:

• Enable FPU (floating-point unit) access so we can do non-integer mathematics
• Switch from EL3 (supervisor exception level) down to EL1 (kernel exception level), disabling the MMU all the same
• Move the addresses for the spin_cpu variables to accommodate a larger boot.S
• Implement a get_el function to check which exception level we're at (for debug mainly)

Work in progress!

This part is still work in progress - and it's a lot of work - so keep watching this space!

In the meantime, do have a go at building some of the samples...