After another round of bug fixing and the creation of a BMOW keyboard driver, the keyboard works! And it truly works this time: pressing the A key shows “a” on the LCD, and pressing SHIFT and the A key shows “A”, just like you’d expect. The craziness with key scan codes has been conquered. I even added support for newline, backspace, and scrolling to the LCD driver, to create a decently functional 20×4 text editor.

I’m using an old Compaq PS/2  keyboard that I had lying around. With its own keyboard input and LCD output, BMOW is now an interactive, self-contained computer. No connection to a PC or terminal is needed. It’s just psychological, but somehow this makes it feel more “real”.

Writing the keyboard driver wasn’t too bad. I wrote it first in C, then hand-translated the C to assembly. It’s a simple state machine that maintains the current up/down state of all the modifier keys (shift, control, alt), as well as some state related to extended key codes, and make/break flags. When a new scan code arrives, it decides if that represents an internal state change (shift was pressed, etc), or a real key event. If it’s a real key event, it maps the scan code to an ASCII value using the modifier values and a lookup table, stores it in OS keyboard buffer, and sets a flag indicating that a new key is ready for handling by the program.

When I first powered on the CPU to test the keyboard driver, it didn’t work at all. The hardware went back to the annoying state of “nothing happens at all”, which is a pain to debug. After some sleuthing, I traced the cause to a bug in the GAL equations for the stack pointer: the most significant bit of the stack was getting cleared whenever the SP was incremented. Doh! How did that ever work before? After fixing the stack pointer bug, the keyboard still didn’t work. With more sleuthing, I discovered that two of the wire connections on the keyboard connector had broken off. Double doh!

Oddly, having a working text “editor” with the keyboard and LCD seems less interesting than the scan code debug display I had before. Typing text on a keyboard and seeing it on a screen is something we do all the time, and it seems completely unremarkable. We just expect the right letters to appear, backspace to work, and return to begin a new line, without even thinking about it. I guess BMOW has now reached the same level of dull predictability as any other PC!