Hal Abelson, "Turtle Geometry" (1979)

turtle_geometry.asm

;  Logo Language Interpreter for the Apple-II-Plus Personal Microcomputer

;  Written and developed by Stephen L. Hain, Patrick G. Sobalvarro, and
;  Leigh L. Klotz with the M.I.T. Logo Group under the supervision of Hal
;  Abelson at the Massachusetts Institute of Technology.

;  Property of the M.I.T. Logo Laboratory,
;  545 Technology Square, Cambridge, MA 02139.
;  Copyright (C) 1980 Massachusetts Institute of Technology
;  All rights reserved.

;  This version of the Logo Language Interpreter requires an Apple-II-Plus
;  microcomputer with the Autostart ROM monitor, a full 48 kilobytes of
;  random access memory, a floppy disk, and the Apple Language System card.
.PAGE
.SBTTL		Turtle-Graphics Utility Routines

;Graphics init routine: set up scrunch factor to GRPHK1.
GRINIT:	SETNUM SCRNCH,GRPHK1
	RTS

GSTART:	LDA INPFLG
	BNE GRIGN	;Ignore primitive if in eval-loop
	LDA GRPHCS	;Checks to see if Graphics mode
	BMI SDRAW3
SDRAW1:	LDX #$00	;Set up default palette and pencolor
	STX PALETN	;(Double width white on black)
	LDA CLTAB1
	STA BKGND
	INX
	STX COLNUM
	LDA CLTAB1+1	;Default is pencolor 1
	STA PNCOLR
	JSR NOEDBF	;Buffer is not retrievable
	LDA PRMPAG	;Primary page