Monday, 26 June 2017

65C816... Meh!

Another first today - my first 65C816 program. I purposefully omitted the exclamation mark from that last sentence because it really is nothing to get excited about. In fact, if you've never written 65C816 code before, don't rush to change that.

I've replaced the apple.asm file in the Asteroids project with another named iigs.asm. Currently the startup code enables the Super High Resolution (SHR) display, sets linear mapping mode, enables shadowing, and then switches to full 16-bit mode to initialise the palette (all 2 colours in one of 16 palettes) and the SCB. The frame rendering code simply switches to 16-bit mode, then immediately back to 8-bit mode before returning to the Asteroids code.

Booting the disk under IIGS emulation eventually - after the machine boots itself and the floppy disk image - results in a black screen. I've also verified that the game is running and the frame renderer is repeatedly being called. And writing values to the SHR memory from the MAME debugger results in pixels appearing on the display. So no crashes so far...

As for the 65C816; no 8-bit memory accesses in 'full' 16-bit mode? OK, perhaps not so much of an issue if the machine is designed from the ground up around the CPU, but when you're running on an architecture with byte-wide softswitches... and interfacing to 8-bit code and data structures... you're in for a bad time.

Then there's the issue, for example, with the assembler not unambiguously knowing whether to load the accumulator with an 8-bit or 16-bit immediate value - because the mnemonics (and, incidentally, the opcode values) are identical. You have to give it hints, and hope it gets it right. A recipe for frustrating bugs if ever I've seen one.

Anyway, as a first pass, I'll be replicating the logic in the text version, and parsing the VDG display list in the same way. I suspect all the parsing code will remain (8-bit) 6502, and I'll only switch into 16-bit mode to render the bitmaps to the SHR. But first I need to prepare said bitmaps for the IIGS display.

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