WIP alternate imul16

not working at present

.mailmap

@@ -1,2 +0,0 @@
-Brooke Vibber <bvibber@pobox.com>
-Brooke Vibber <bvibber@pobox.com> <brion@pobox.com>

Makefile

@@ -2,8 +2,8 @@
 
 all : mandel.xex
 
-mandel.xex : mandel.o tables.o atari-asm-xex.cfg
-	ld65 -C ./atari-asm-xex.cfg -o $@ mandel.o tables.o
+mandel.xex : mandel.o tables.o
+	ld65 -C ./atari-asm-xex.cfg -o $@ $+
 
 %.o : %.s
 	ca65 -o $@ $<

atari-asm-xex.cfg

@@ -1,28 +0,0 @@
-FEATURES {
-    STARTADDRESS: default = $2E00;
-}
-SYMBOLS {
-    __STARTADDRESS__: type = export, value = %S;
-}
-MEMORY {
-    ZP:      file = "", define = yes, start = $0082, size = $007E;
-    MAIN:    file = %O, define = yes, start = %S,    size = $4000 - %S;
-    # Keep $4000-7fff clear for expanded RAM access window
-    TABLES:  file = %O, define = yes, start = $8000, size = $a000 - $8000;
-    # Keep $a000-$bfff clear for BASIC cartridge
-}
-FILES {
-    %O: format = atari;
-}
-FORMATS {
-    atari: runad = start;
-}
-SEGMENTS {
-    ZEROPAGE: load = ZP,      type = zp,  optional = yes;
-    EXTZP:    load = ZP,      type = zp,  optional = yes; # to enable modules to be able to link to C and assembler programs
-    CODE:     load = MAIN,    type = rw,                  define = yes;
-    RODATA:   load = MAIN,    type = ro   optional = yes;
-    DATA:     load = MAIN,    type = rw   optional = yes;
-    BSS:      load = MAIN,    type = bss, optional = yes, define = yes;
-    TABLES:   load = TABLES,  type = ro,  optional = yes, align = 256;
-}

readme.md

@@ -14,37 +14,30 @@ Non-goals:
 
 Enjoy! I'll probably work on this off and on for the next few weeks until I've got it producing fractals.
 
--- brooke, january 2023 - december 2024
+-- brion, january 2023
 
 ## Current state
 
-Basic rendering is functional, with interactive zoom/pan (+/-/arrows) and 6 preset viewports via the number keys.
+Basic rendering is functional, but no interactive behavior (zoom/pan) or benchmarking is done yet.
 
-The 16-bit signed integer multiplication takes two 16-bit inputs and emits one 32-bit output in the zero page, using the Atari OS ROM's floating point registers as workspaces. Inputs are clobbered.
+The 16-bit signed integer multiplication works; it takes two 16-bit inputs and emits one 32-bit output in the zero page, using the Atari OS ROM's floating point registers as workspaces. Inputs are clobbered.
 
-* 16-bit multiplies are decomposed into 4 8-bit unsigned multiplies and some addition
-* an optimized case for squares uses a table of 8-bit squares to reduce the number of 8-bit multiplication sub-ops
-* when expanded RAM is available as on 130XE, a 64KB 8-bit multiplication table accelerates the remaining multiplications
-* without expanded RAM, a table of half-squares is used to implement the algorithm from https://everything2.com/title/Fast+6502+multiplication
+The main loop is a basic add-and-shift, using 16-bit adds which requires flipping the sign of negative inputs (otherwise you'd have to add all those sign-extension bits). Runs in 470-780 cycles depending on input.
 
-The mandelbrot calculations are done using 3.13-precision fixed point numbers with 6.26-precision intermediates.
+The mandelbrot calculations are done using 4.12-precision fixed point numbers. It may be possible to squish this down to 3.13.
 
 Iterations are capped at 255.
 
-The pixels are run in a progressive layout to get the basic shape on screen faster.
+## Next steps
 
-There is a running counter of ms/px using the vertical blank interrupts as a timer, used to track our progress. :D
+Add a running counter of ms/px using the vertical blank interrupts as a timer. This'll show how further work improves it!
 
-There's a check for cycles in (zx,zy) output when in the 'lake'; if values repeat, they cannot escape. This is a big time saver in fractint.
+Check for cycles in (zx,zy) output when in the 'lake'; if values repeat, they cannot escape. This is a big time saver in fractint.
 
-There's some cute color cycling.
+I may be able to do a faster multiply using tables of squares for 8-bit component multiplication.
 
 ## Deps and build instructions
 
 I'm using `ca65` as a macro assembler, and have a Unix-style `Makefile` for building. Should work fairly easily on Linux and Mac. Might work on "raw" Windows but I use WSL for that.
 
 Currently produces a `.xex` executable, which can be booted up in common Atari emulators and some i/o devices.
-
-## Todo
-
-See ideas in `todo.md`.

tables.js

@@ -11,40 +11,23 @@ function db(func) {
     return lines.join('\n');
 }
 
-let squares = [];
-for (let i = 0; i < 512; i++) {
-    squares.push(Math.trunc((i * i + 1) / 2));
-}
-
 console.log(
 `.segment "TABLES"
 
 .export mul_lobyte256
 .export mul_hibyte256
 .export mul_hibyte512
-.export sqr_lobyte
-.export sqr_hibyte
 
-; (i * i + 1) / 2 for the multiplier
 .align 256
 mul_lobyte256:
-${db((i) => squares[i] & 0xff)}
+${db((x) => Math.round(x * x / 2) & 0xff)}
 
 .align 256
 mul_hibyte256:
-${db((i) => (squares[i] >> 8) & 0xff)}
+${db((x) => (Math.round(x * x / 2) >> 8) & 0xff)}
 
 .align 256
 mul_hibyte512:
-${db((i) => (squares[i + 256] >> 8) & 0xff)}
-
-; (i * i) for the plain squares
-.align 256
-sqr_lobyte:
-${db((i) => (i * i) & 0xff)}
-
-.align 256
-sqr_hibyte:
-${db((i) => ((i * i) >> 8) & 0xff)}
+${db((x) => (Math.round((x + 256) * (x + 256) / 2) >> 8) & 0xff)}
 
 `);

todo.md

@@ -1,17 +0,0 @@
-things to try:
-
-* fix status bar to show elapsed time, per-iter time, per-pixel iter count
-
-* 'turbo' mode disabling graphics in full or part
-
-* patch the entire expanded-ram imul8xe on top of imul8 to avoid the 3-cycle thunk penalty :D
-
-* maybe clean up the load/layout of the big mul table
-
-* consider alternate lookup tables in the top 16KB under ROM
-
-* y-axis mirror optimization
-
-* extract viewport for display & re-input via keyboard
-
-* fujinet screenshot/viewport uploader