Automatic Proofreader by Charles Brannon for the Commodore 64 Compute Gazette October 1983

Methodology

Text entered has it's PETSCII values summed. The LSB of the sum is printed in reverse print in the upper left corner. The checksum is only a summation so order of text, rotations and other inline swaps don't modify the checksum. The tool is launched by intercepting the CHRIN kernel call by modifying the vector at 0x0324-0x0325 to be instead a location within this program, which itself calls CHRIN, but also collects the keystrokes in the process to calculate the checksum, with special cases for SPACE and RETURN.

Code

Line number	BASIC command	Comment
100	FOR I=828 TO 932:	I is counting through tape buffer
	READ A:	Example
	POKE I,A:	Example
	NEXT:	Example
	PRINT"{CLR}{RVS}sys 828{OFF} to activate.{2 down}"	Instruction text
110	POKE178,165:	Page Zero. Move begin of TAPE BUFFER to 0x03A5 from 0x033C. This substantially shortens the tape buffer.
	print"sys 828{home}":	Prepare text for user. 0d828 = 0x033C
	end	Release back to user from BASIC program mode
120	DATA169,75,141,36,3,169,3,141	A9 4B 8D 24 03 A9 03 8D
130	DATA37,3,169,0,133,255,96,32	25 03 A9 00 85 FF 60 20
140	DATA87,241,133,252,134,253,132,254	57 F1 85 FC 86 FD 84 FE
150	DATA8,201,13,240,17,201,32,240	08 C9 0D F0 11 C9 20 F0
160	DATA5,24,101,255,133,255,165,252	05 18 65 FF 85 FF A5 FC
170	DATA166,253,164,254,40,96,169,13	A6 FD A4 FE 28 60 A9 0D
180	DATA32,210,255,165,214,141,165,3	20 D2 FF A5 D6 8D A5 03
190	DATA206,165,3,169,0,133,216,169	CE A5 03 A9 00 85 D8 A9
200	DATA19,32,210,255,169,18,32,210	13 20 D2 FF A9 12 20 D2
210	DATA255,169,58,32,210,255,166,255	FF A9 3A 20 D2 FF A6 FF
220	DATA169,0,32,205,189,169,0,133	A9 00 20 CD BD A9 00 85
230	DATA255,169,32,32,210,255,32,210	FF A9 20 20 D2 FF 20 D2
240	DATA255,173,165,3,133,214,76,98,3	FF AD A5 03 85 D6 4C 62 03

Assembly

Move the vector to CHRIN into this program and then terminate

033C : LDA Immediate 4B ; LSB of new vector for CHRIN (A9 4B)

033E : STA Absolute 0x0324 ; LSB of CHRIN(8D 24 03)

0341 : LDA Immediate 03 ; MSB of new vector for CHRIN (A9 03)

0343 : STA Absolute 0x0325 ; MSB of CHRIN (8D 25 03)

Clear checksum register

0346 : LDA Immediate 00 ; Clear A (A9 00)

0348 : STA Zeropage FF ; Clear 0x00FF (85 FF)

Terminate

034A : RTS ; 60

This is now resident code that is executed as part of CHRIN

Launch CHRIN

034B : JSR F157 ; Kernel routine CHRIN (20 57 F1)

Undocumented register dump for X, Y

034E : STA Zeropage FC ; MLZP Memory 2 (85 FC)

0350 : STX Zeropage FD ; MLZP Memory 3 (86 FD)

0352 : STY Zeropage FE ; MLZP Memory 4 (84 FE)

0354 : PHP ; 08

0355 : CMP Immediate 0D ; Is this a CR? (C9 0D)

0357 : BEQ 11 ; If so, go to 0x036A (F0 11)

0359 : CMP Immediate 20 ; Is this an ASCII space? (C9 20)

035B : BEQ 05 ; If so, go to 0x0362 (F0 05)

035D : CLC ; 18

035E : ADC Zeropage FF ;65 FF

0360 : STA Zeropage FF ;85 FF

Is Space

0362 : LDA Zeropage FC ;A5 FC

0364 : LDX Zeropage FD ;A6 FD

0366 : LDY Zeropage FE ;A4 FE

0368 : PLP ; 28

0369 : RTS ; 60

It is CR

036A : LDA Immediate 0D ; PETSCII Carriage return to next line A9 0D

036C : JSR FFD2 ; CHROUT New Line (20 D2 FF)

capture current line number. We're about to print and will want to return to this location

036F : LDA Zeropage D6 ; A5 D6

0371 : STA Absolute 03A5 ; 8D A5 03

0374 : DEC Absolute 03A5 ; CE A5 03

Line number stored in 0x03A5. Get ready to print to screen

0377 : LDA Immediate 00 ; (A9 00)

0379 : STA Zeropage D8 ; Inserts = 0 (85 D8)

037B : LDA Immediate 13 ; HOME character (A9 13)

037D : JSR FFD2 ; CHROUT - Set cursor to pos 0,0 (20 D2 FF)

0380 : LDA Immediate 12 ; Reverse character code (A9 12)

0382 : JSR FFD2 ; CHROUT - Set text to reverse mode (20 D2 FF)

0385 : LDA Immediate 3A ; ASCII":" (A9 3A)

0387 : JSR FFD2 ; Print the ":" (20 D2 FF)

038A : LDX Zeropage FF ; Read Checksum from FF (A6 FF)

038C : LDA Immediate 00 ; Force output to be 8 bit number (A9 00)

038E : JSR BDCD ; LINPRT - Print AAXX to screen (20 CD BD)

0391 : LDA Immediate 00 ; Clear A (A9 00)

0393 : STA Zeropage FF ; Clear Checksum to zero (85 FF)

0395 : LDA Immediate 20 ; ASCII Space (A9 20)

0397 : JSR FFD2 ; CHROUT - Draw two spaces (20 D2 FF)

039A : JSR FFD2 ; CHROUT (20 D2 FF)

039D : LDA Absolute 03A5 ; AD A5 03

03A0 : STA Zeropage D6 ; Current physical line number 0-24 (85 D6)

03A2 : JMP Absolute 0362; 4C 62 03

03A5 : xx ; Line number, used by program