more documentation of similarities and avoiding them where practicable.

c64/kernal/chrout.s

@@ -88,12 +88,13 @@ no_need_to_extend:
 	dey
 	cpy current_screen_x
 	bne -
-	;; Put space in the inserted gap
-	lda #$20
-	sta (current_screen_line_ptr),y
 
 	;; Increase insert mode count (which causes quote-mode like behaviour)
 	inc insert_mode
+	
+	;; Put space in the inserted gap
+	lda #$20
+	sta (current_screen_line_ptr),y
 
 	jmp chrout_done
 not_94:	
@@ -566,10 +567,10 @@ scroll_copy_loop2:
 	;; Fill in scrolled up area
 	ldx load_or_scroll_temp_pointer+0
 scroll_copy_loop3:
-	lda #$20
-	sta (current_screen_line_ptr),y
 	lda text_colour
 	sta (current_screen_line_colour_ptr),y
+	lda #$20
+	sta (current_screen_line_ptr),y
 	iny
 	dex
 	bne scroll_copy_loop3

c64/kernal/cint.s

@@ -26,18 +26,18 @@ cint:
 	;; Making some numbers up here: Repeat every ~1/10th sec
 	;; But require key to be held for ~1/3sec before
         ;; repeating (Compute's Mapping the 64 p58)
-	lda #22-2 		; Fudge factor to match speed
-	sta key_first_repeat_delay
+	ldx #22-2 		; Fudge factor to match speed
+	stx key_first_repeat_delay
 
 	;; Set current colour for text (Compute's Mapping the 64 p215)
-	lda #$0e  		; light blue by default
-	sta $0286
+	ldx #$0e  		; light blue by default
+	stx $0286
 
 	;; Set maximum keyboard buffer size (Compute's Mapping the 64 p215)
-	lda #10
-	sta key_buffer_size
+	ldx #10
+	stx key_buffer_size
 	;; Put non-zero value in enable_case_switch
-	sta enable_case_switch
+	stx enable_case_switch
 	
 	;; Fallthrough/jump to screen clear routine (Compute's Mapping the 64 p215)
 	jmp clear_screen

src/similarity.c

@@ -108,6 +108,12 @@ int main(int argc,char **argv)
 	// Ignore matches of no more than 3 bytes,
 	// since that is the max length of a 6502 instruction
 	int single_instruction=0;
+	if (k==5) {
+	  if (f1[i]==0xa9&&f1[i+2]==0x8d) break; // LDA #$xx / STA $nnnn
+	}
+	if (k==4) {
+	  if (f1[i]==0xa9&&f1[i+2]==0x85) break; // LDA #$xx / STA $nn
+	}
 	if (k==3) {
 	  // Reject some very common instruction fragments
 
@@ -118,6 +124,12 @@ int main(int argc,char **argv)
 	  if (f1[i]==0xb0) break;
 	  if (f1[i]==0x90) break;
 	  if (f1[i]==0x10) break;
+	  // Any single byte followed by a branch
+	  if (f1[i+1]==0xd0) break;
+	  if (f1[i+1]==0xf0) break;
+	  if (f1[i+1]==0xb0) break;
+	  if (f1[i+1]==0x90) break;
+	  if (f1[i+1]==0x10) break;
 	  // Any 2-byte opcode followed by a branch
 	  if (f1[i+2]==0xd0) break;
 	  if (f1[i+2]==0xf0) break;

strings/18692885

@@ -0,0 +1,5 @@
+CLC / ADC #40 / STA $nn
+
+Add 40 to a ZP location.  No creativity here. 
+Effectivelhy only one insstruction.
+Not copyrightable.

strings/424153494320

@@ -0,0 +1,4 @@
+string "BASIC "
+
+Since we are writing a BASIC language interprettor, such a string is expected.
+BASIC appears in many other works. It cannot be copyrighted as a word.

strings/4241534943205632

@@ -0,0 +1,8 @@
+String "BASIC V2"
+
+As we are implementing a V2 class BASIC interprettor, it is 
+natural for us to include this string.
+There are many BASIC intreprettors, and the word cannot be
+copyrighted, nor it is reasonable to contemplate that a party
+could be constrained from having a BASIC with version 2, or
+displaying that information anywhere. Not copyrightable.

strings/60AD8D02

@@ -0,0 +1,7 @@
+RTS + LDA $028D
+
+RTS is a very common instruction, as is loading A from absolute address.
+The RTS is the end of a routine, and the LDA $028D thus the start of
+another. Thus this would occur whenever a routine begins with loading
+A from $028D, immediately after another routine.  Nothing creative.
+Not copyrightable.

strings/7702E8

@@ -0,0 +1,3 @@
+Something with $0277 / INX
+
+Only one whole instruction here. Not copyrightable.

strings/860291F3

@@ -0,0 +1,3 @@
+something with $0286 / STA ($F3),Y
+
+Only one whole instruction. Not copyrightable.

strings/88B1D1C891D188

@@ -0,0 +1,44 @@
+Copy screen + colour RAM to the right one place
+
+This sequence of instructions:
+
+loop:
+       DEY
+       LDA ($D1),Y
+       INY
+       STA ($D1),Y
+       DEY
+       LDA ($F3),Y
+       INY
+       STA ($F3),Y
+       DEY
+       CPY $D3
+       BNE loop
+       LDA #$20
+       STA ($D1),Y
+
+is the obvious any minimal way to copy the screen and colour RAM data.
+The constants $D1 and $F3 are fixed by the publicly documented interface
+of the KERNAL to point to the screen and colour RAM of the screen.
+
+The Y register is the only register that can be used to post-increment an
+indirect address on the 6502.
+
+Thus the only efficient manner to copy a string to the right one byte
+is to read, increment Y by one to get the destination address, store,
+then decrement Y again so that it points to the source offset, now read
+the colour RAM byte, add one to Y so that we point to the destination,
+and then write it.  
+
+Similarly the check against $D3 (current column of cursor on screen) is also
+necessary for working out when to terminate the loop.  The storing of $20
+into the screen RAM is also required to effect the insertion of a space.
+
+In fact, about the only variant possible would be to swap the $D1 and $F3
+constants so that the colour RAM is copied before the screen RAM in each
+loop, instead of after it.
+
+It is telling that this routine was the result of independent implementation
+of this routine, without looking at the KERNALs routine, in terms of supporting
+the argument that the routine is the obvious manner in which to do this.
+

strings/88B1F3C891F388

@@ -0,0 +1,44 @@
+Copy screen + colour RAM to the right one place
+
+This sequence of instructions:
+
+loop:
+       DEY
+       LDA ($D1),Y
+       INY
+       STA ($D1),Y
+       DEY
+       LDA ($F3),Y
+       INY
+       STA ($F3),Y
+       DEY
+       CPY $D3
+       BNE loop
+       LDA #$20
+       STA ($D1),Y
+
+is the obvious any minimal way to copy the screen and colour RAM data.
+The constants $D1 and $F3 are fixed by the publicly documented interface
+of the KERNAL to point to the screen and colour RAM of the screen.
+
+The Y register is the only register that can be used to post-increment an
+indirect address on the 6502.
+
+Thus the only efficient manner to copy a string to the right one byte
+is to read, increment Y by one to get the destination address, store,
+then decrement Y again so that it points to the source offset, now read
+the colour RAM byte, add one to Y so that we point to the destination,
+and then write it.  
+
+Similarly the check against $D3 (current column of cursor on screen) is also
+necessary for working out when to terminate the loop.  The storing of $20
+into the screen RAM is also required to effect the insertion of a space.
+
+In fact, about the only variant possible would be to swap the $D1 and $F3
+constants so that the colour RAM is copied before the screen RAM in each
+loop, instead of after it.
+
+It is telling that this routine was the result of independent implementation
+of this routine, without looking at the KERNALs routine, in terms of supporting
+the argument that the routine is the obvious manner in which to do this.
+

strings/8D02C903D0

@@ -0,0 +1,4 @@
+something with $028D / CMP #$03 / BNE somewhere
+
+Only one whole instruction. Nothing creative.
+Not copyrightable.

strings/8D8C02A90E8D8602A9

@@ -0,0 +1 @@
+STA $028C / LDA #$0E / STA $0286 / LDA 

strings/A6C6EC8902

@@ -0,0 +1,3 @@
+Use X register to compare contents of a ZP and absolute address location
+
+Nothing copyrightable here in this simple comparison of memory contents.

strings/B1D1C920

@@ -0,0 +1,4 @@
+LDA ($D1),Y / CMP #$20
+
+Compare a byte read from a pointer with a specific value.
+No creativity here. Not copyrightable.

strings/E6C911D0

@@ -0,0 +1,3 @@
+Fragment / CMP #$11 / BNE somewhere
+
+Only one whole intruction here. Not copyrightable.

strings/E6C913D0

@@ -0,0 +1,3 @@
+Fragment / CMP #$13 / BNE somewhere
+
+Only one whole intruction here. Not copyrightable.