COMFY-6502: a slight correction

After all the work to write a post showing off COMFY-6502’s ability to reproduce the Red Book tone routine, I made a slight mistake in translation. The code I presented differed ever-so-slightly from the Red Book routine in the case where the Y-register reached zero.

In the original, when the Y-register reaches zero, the DEX is still executed. In the code I posted, the expiring “duration tick” skips a count on the pitch. A minor error, but one I should own up to.

Here’s a better effort. COMPILE-SYMBOLIC contains a MACROLET defining MODULE as a call to COMFY-6502:COMPILE.

(compile-symbolic
  (equ speaker #xc030)
  (equ duration 1)
  (equ pitch 0)
  (module
   (comfy-6502:loop
      ;; repeat until some clause loses: actually, only exit is
      ;; through return, so each clause should be ensured of winning.
    (LDX pitch)
    (LDA speaker)
    (not
     (comfy-6502:loop  ;; repeat until whap time
     DEY
     (not (seq zero? (DEC duration) zero? return))
     (seq DEX (not zero?)))))))

resulting in

(; reload-pitch
 (LDX :ZERO-PAGE) (:ZERO-PAGE 0)
 (LDA :ABSOLUTE) (:ABSOLUTE 49200)
 ; spin
 (DEY)
 (BNE) (:BRANCH 6) ; not-duration-tick
 (DEC :ZERO-PAGE) (:ZERO-PAGE 1)
 (BNE) (:BRANCH 2)
 (RTS)
 ; not-duration-tick
 (DEX)
 (BEQ) (:BRANCH 4) ; goto-reload-pitch
 (JMP :ABSOLUTE) (:LONG-BRANCH -12) ; spin
 ; goto-reload-pitch
 (JMP :ABSOLUTE) (:LONG-BRANCH -20) ; reload-pitch
)

For fun, I tried to get the code to emit the RTS at the very end, as in the original.

(compile-symbolic
  (equ speaker #xc030)
  (equ duration 1)
  (equ pitch 0)
  (module
    (alt (loop (seq (LDX pitch) (LDA speaker))
            (not (while
                    (seq DEY (not (seq zero? (seq (DEC duration) zero?)))
                    (seq DEX (not zero?)))))
          RTS)))

(; reload
  (LDX :ZERO-PAGE) (:ZERO-PAGE 0)
  (LDA :ABSOLUTE) (:ABSOLUTE 49200)
  ; spin
  (DEY)
  (BNE) (:BRANCH 5) ; not-duration
  ; duration-tick
  (DEC :ZERO-PAGE) (:ZERO-PAGE 1)
  (BEQ) (:BRANCH 10) ; exit
  ; not-duration
  (DEX)
  (BEQ) (:BRANCH 4) ; goto-reload
  ; goto-spin
  (JMP :ABSOLUTE) (:LONG-BRANCH -11)
  ; goto-reload
  (JMP :ABSOLUTE) (:LONG-BRANCH -19) ; reload
 ; exit
  (RTS))

“Loop tightening” could change (BEQ) (:BRANCH 4) to (BEQ) (:BRANCH -14); the (JMP :ABSOLUTE) (:LONG-BRANCH -19) is then never used, and could be omitted, changing (BEQ) (:BRANCH 10) to (BEQ) (:BRANCH 7).

“Loop compaction” could potentially recognize (BEQ) (:BRANCH -14) (JMP :ABSOLUTE) (:LONG-BRANCH -11), where no other instruction references the JMP, could be converted to (BEQ) (:BRANCH -14) (BNE) (:BRANCH -11), shortening the (BEQ) (:BRANCH 7) one byte more to (BNE) (:BRANCH 6).

Note some interesting equivalents I should probably implement as COMFY macros.

(if A B C) where C is do-nothing-but-win is (not (seq A (not B)). If A wins, B is executed to determine the result of the form. If A loses, the if wins. I will probably call this (when A B).

(if A B C) where B is a do-nothing-but-win is (alt A C), where alt is the “dual” of seq. (alt A B C …) == (not (seq (not A) (not B) (not C) …). For the two-argument case, I would call this (unless A B).

The one twist I can think of is to add an “implied SEQ” to these forms. (when A B C …) would be (when A (seq B C …)). (unless A B C …) would be (unless A (seq B C …)).

Tags: 6502, COMFY, Red book tone routine