Tuesday, 18 July 2017

A token effort

I have to admit, I haven't been able to tear myself away from the C port to make any further progress on the IIGS port. However, it hasn't all been for nought as it has definitely reinforced my understanding of the arcade code, and cemented my decision regarding tokenising (optimising) the display list for the 8-bit ports.

Before I get to that; most of the work on the C code has been 'infrastructure work' and low-level DVG interface routines, which necessarily support both the new abstract display list and the original in parallel - to facilitate debugging and development. What that leaves, then, is the game logic and housekeeping code which generally tends to be easier to translate to C; the upshot of all this is that I don't think the C port is going to take very long to complete at all!

[Just for the record, I have the C port rendering all the text, including scores, and rendering and animating the asteroids themselves. The pseudo-random number generator is also in lock-step with the arcade machine and produces the same output at the appropriate times].

Keep in mind that the arcade code is only 6KB of 6502 - a lot of that munging 16-bit numbers - and it's not surprising that the C port isn't huge. From memory, Knight Lore was ~12KB of Z80 code and translated to ~5K lines of C. I'm around 1,300 lines for Asteroids already, and you could estimate it'll be in the vicinity of 2,500 lines.

Getting back to the IIGS (and 8-bit) ports; aside from the existing CUR (which sets the current beam coordinates) and HALT display list commands, there'll be a distinct command for the rendering of each object in the game, comprising character, extra ship, copyright, asteroid, ship, saucer, shot, shrapnel and exploding ship. I may add one last command to set the brightness - something the arcade code does but Norbert doesn't bother with - simply because the IIGS has the palette to support some variance in brightness.

Some of those commands will have one or two parameters, but all will render at the current beam position. The parameters will be succinct and optimised for the bitmap display routines. What this means is that I can actually remove a lot of code that generates the display list content that is irrelevant for the port, such as DVG subroutine calls or component vector commands. This is one area where I'll be able to improve performance over Norbert's emulators, only because I effectively have the arcade 6502 source that I can modify and re-assemble at will.

I've also identified which of the bitmaps will and won't require bit-shifting, and which will require an extra byte's width to do so. Because, for example, all of the game's text message coordinates are fixed, specified on a 0-255 grid (before being scaled-up in the display list), and also happen to have even X coordinates, I don't have to bit-shift any of the character set for the IIGS 2BPP SHR graphics!

Most of the remaining bitmaps will require bit-shifting, and a few - not all - of those will require an additional byte's width to facilitate it. But that simply boils down to an extra compare and load for each object rendering, unless I need to really wring the performance out of the rendering routines.

My next task now is to generate shifted bitmap data, which is trivial, and essentially start over from scratch with the IIGS port. I'll probably have to stub out all the routines that write to the display list, and then begin work on the so-called tokenising version. None of that should be too difficult...

[UPDATE: I've regenerated the 6502 ASM file from my disassembly, starting the IIGS port from scratch. All of the DVG write routines have been stubbed-out so that only the CUR command is now written to display list. Next is tokenising the character command and then rendering it on the IIGS.]

As for the erasure; I'm planning on (eventually) making use of the ping-pong display list buffer. Immediately before rendering the new list, I'll simply re-parse the old buffer and use it essentially as dirty rectangles. I do have more sophisticated optimisation possibilities up my sleeve; it's useful to know, for example, that all objects are written to the display list in a fixed order. I'll leave all that, however, until I need it - if ever.

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