1 * String Comparison Routine for PASCAL strings
    2 * INPUT *
    3 * Pointer to First String in STRING1, STRING1+1
    4 * Pointer to Second String in STRING2, STRING2+1
    5 * STRING1 and STRING2 must be located on the ZERO PAGE
    6 * TEMP can be anywhere in memory
    7 * OUTPUT *
    8 * If String1 < String2 then carry clear; else carry set
    9 * If String1 = String 2 then Z set; else Z clear
   10 * Destroys A-REG and Y-REG; X-REG unchanged
   11 STRING1  EQU   6
   12 STRING2  EQU   8
   13 TEMP     EQU   9
   14 PSTRCMP  LDY   #0
   15          LDA   (STRING1),Y
   16          CMP   (STRING2),Y
   17          BCC   :1
   18          LDA   (STRING2),Y
   19 :1       STA   TEMP
   20          LDY   #1
   21 :2       LDA   (STRING1),Y
   22          CMP   (STRING2),Y
   23          BNE   :3
   24          INY
   25          CPY   TEMP
   26          BCC   :2
   27          LDY   #0
   28          LDA   (STRING1),Y
   29          CMP   (STRING2),Y
   30 :3       RTS

T0 = $00
T1 = $02
T2 = $04

strcmp:
   ; T0 = input string 1 (255 chars or less)
   ; T1 = input string 2 (255 chars or less)
   ;
   ; on exit, flags should be set as expected for CMP.
   ;  C flag clear if string 1 < string 2
   ;  Z flag set if string 1 = string 2
   ;
   ; I treat all registers as effectively destroyed.
   ;  A is trashed outright.
   ;  X is untouched, but should never depend on that anyway.
   ;  Y contains length of shortest string, less the NULL.
   ;    (same semantics as strlen()).
 

   ldy #$00

.loop:
   lda (T1),y
   beq .different   ; Terminating NULLs always break the loop
   lda (T0),y
   beq .different   ; Terminating NULLs always break the loop
   cmp (T1),y
   bne .different   ; Mismatches also break the loop
   iny
   jmp .loop

.different:
   lda (T0),y   ; reset the flags accordingly
   cmp (T1),y
   rts

;
; As a bonus: strlen -- get length of string, excluding NULL byte.
; Since we're relying on strcmp() to get the length, it's not as fast
; as a native implementation of strlen(), and it destroys T1 too.
;

strlen:
   lda T0
   sta T1
   lda T0+1
   sta T1+1
   jmp strcmp

   LDY #0
   LDA (STRING1),Y
   CMP (STRING2),Y
   BCC :1
:1 LDA (STRING2),Y
   TAX
   BEQ :3
:2 INY
   LDA (STRING1),Y
   CMP (STRING1),Y
   BNE :3
   DEX
   BNE :2
   LDA (STRING1,X)
   CMP (STRING2,X)
:3 RTS

   LDY #0
:1 LDA (STRING1),Y
   BEQ :2
   CMP (STRING2),Y
   BNE :3         
   INY             ; note: C=1 since CMP was equal
   BNE :1
   INC STRING1+1
   INC STRING2+1
   BCS :1          ; always branches
:2 CMP (STRING2),Y
:3 RTS