The code at the end runs fine (it doesn't really matter what it does, but some forum members might spot some of the style :) )

(Don't know why the layout is awful - it's OK in Studio)

Can I ask some/all to:

- load it into Studio and run it in the simulator. Put a breakpoint at the line after DONE: and run it, Note that everytime you do, r18 = 0x78 and r19 = 0x35, just as required. Single step the code and watch the PORTB output - just what you'd expect

- Now download it to an 8515 (or an m16, if you change the .include - it should work OK) in an STK500. Watch the LEDs. Do they indicate what they should (0x78)? Press the Reset button on the STK500. Watch the LEDs - are they the same? Press the Reset button on the STK500. Watch the LEDs - are they the same? Press the Reset button on the STK500. Watch the LEDs - are they the same?

On mine I get a different set of LEDs illuminated each time. But the processor is being reset each time, so what is causing the different results?

.include "m8515def.inc" .DEF ZERO = R15 ;USED AS IMMEDIATE ZER0 .DEF X1 = R16 ;X1 .DEF X2 = R17 ;X2 AND DELTA-X .DEF Y1 = R18 ;<== FINAL ANSWER HERE .DEF Y1H = R19 .DEF Y2 = R20 ;Y2 AND DELTA-Y .DEF Y2H = R21 .DEF Y2HH = R22 .DEF IX = R23 ;GIVEN X AND X-OFFSET .DEF TMP = R24 ;WORKSPACE .DEF TMPH = R25 .DEF N = R26 ;LOOP COUNTER .SET X1_VALUE = 24 .SET X2_VALUE = 50 .SET Y1_VALUE = 9454 .SET Y2_VALUE = 14241 .SET X_VALUE = 47 ;PORTB initialisation - visual debug ser r16 out ddrb, r16 ldi X1 ,X1_VALUE ;LOAD TEST DATA ldi X2 ,X2_VALUE ldi Y1 ,LOW(Y1_VALUE) ldi Y1H,HIGH(Y1_VALUE) ldi Y2 ,LOW(Y2_VALUE) ldi Y2H,HIGH(Y2_VALUE) ldi IX ,X_VALUE clt ;NEG-SLOPE FLAG clr ZERO ;USED AS A ZERO sub Y2,Y1 ;CALC dY INTO Y2 sbc Y2H,Y1H breq DONE ;IF Y1=Y2? WE'RE DONE! brcc NOTNEG ;GO IF SLOPE POSITIVE set ;NEGATIVE SLOPE.. neg Y2 ;CONVERT TO POSITIVE adc Y2H,ZERO neg Y2H NOTNEG: sub X2,X1 ;CALC dX INTO X2 breq DONE ;IF X1=X2? WE'RE DONE! sub IX,X1 ;CALC (X-X1) INTO IX breq DONE ;IF X=X1? WE'RE DONE! cp IX,X2 ;DOES X=X2? breq Y2OUT ;YES? THEN ANS IS Y2! INTERP: movw TMP,Y2 ;MULTIPLY dY(X-X1) mul TMP,IX movw Y2,R0 mul TMPH,IX add Y2H,R0 adc Y2HH,R1 adc Y2HH,ZERO ldi N,16 ;CALC dY(X-X1)/dX LUPIE: rol Y2 rol Y2H rol Y2HH cp Y2HH,X2 brcs SKPSUB DOSUB: sub Y2HH,X2 clc SKPSUB: dec N brne LUPIE rol Y2 rol Y2H DVDONE: com Y2 com Y2H brts SLPNEG ;NEG-SLOPE? add Y1,Y2 ;ANS=Y1+dY(IX-X1)/dX adc Y1H,Y2H rjmp DONE SLPNEG: sub Y1,Y2 ;ANS=Y1-dY(IX-X1)/dX sbc Y1H,Y2H rjmp DONE Y2OUT: movw Y1,Y2 ;ANS=Y2 DONE: mov r16, Y1H com r16 out portb, r16 mov r16, Y1 com r16 out portb, r16 loop: rjmp loop