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base:generating_sines_with_basic

Generating Sines through BASIC

By Doynax

Generating sines in BASIC is slow, but might be suitable for programs that need to be small. Improve if you can!

The routine is currently 28 bytes long and takes 5.9 seconds to execute:

table	= $0400		;; The output is a set of negated (phase-shifted by 180°)
			;; sines between -128 and +127.
			;; Preferably a low page. Must be paged aligned!

loop	lda #<index	;; Load 5-byte float at 'index' into FAC, the fraction of
	ldy #>index	;; which is stepped between -0/256..-255/256.
	jsr $bba2	;; However an integer bias is also added in order to fix
			;; the exponent and make hence it possible to increment the
			;; fraction as a normal binary byte, e.g. a version of the
			;; classic x86 float-to-int conversion trick.
	
	jsr $e277	;; Now calculate sine of FAC. Except skip the initial part
			;; of the BASIC function which divides by 2*PI to get
			;; a fraction out of radians since we've already got one.
			;; The integer bias is taken care by BASIC since sin()
			;; is supposed to be periodic.

	lda #<bias	;; Convert the output in FAC from a float in -1..+1 to a
	ldy #>bias	;; fixed-point value in -128..+127 at the LSB of the
	jsr $b867	;; mantissa by employing the same trick as before of
	lda $65		;; adding a high integer bias.

index	.byte $88	;; This is both a float *and* a piece of code. The exponent
	sta table	;; ($88 corresponds to 2^8) fixes our 8-bit fraction as the
			;; second byte of the mantissa and the STA address' LSB
			;; (don't forget that BASIC floats are big-endian!). And $88
			;; when interpreted as code corresponds to a harmless DEY.
			;; Note that the STA's opcode is an integer part which only
			;; has the effect of negating the index (the sign bit is set).
			;; However the table address' high byte and the subsequent
			;; INC opcode *do* serve as a small offset, shifting the
			;; result by up to one index value. Some might even argue
			;; that placing the table at $8000 would produce the 'proper'
			;; rounding.
	inc *-2
	bne loop

bias	= $befa		;; A float with an exponent of $99 (2^25) and an LSB of
;;	.byte $99	;; zero is used to convert the output to binary. Such byte
;;	.byte $02	;; sequences can be found in six places in the BASIC/Kernal
;;	.byte $01	;; ROMs, at $befa/$bf04/$bf09/$fd53/$fd56/$ff38.
;;	.byte $a9	;; A version with an LSB of $80 would have been useful to
;;	.byte $00	;; create unsigned output (e.g. between $00 and $ff with the
			;; origin at $80) but unfortunately doesn't seem to exist.
			;; Values with different exponents and offsets might be found
			;; to better suit your particular application.
base/generating_sines_with_basic.txt · Last modified: 2015-04-17 04:32 (external edit)