0 ;LIST#D:MYUSR.ASM¢10 ;ASM ,,#D:MYUSR.OBJ¢11 SUM = $D4¢12 NUM1 =$E0¢13 NUM2 = $E2¢20 *=1536 ; Assemble for PAGE 6.¢30 ADDTHEM PLA ; First off the stack is parameter count.¢40 BEQ ERROR ; Always check for no parameters ERROR.¢50 CMP #2 ; Did we get exactly 2 parameters?¢60 BEQ AOK¢70 TAX ; No, clean up stack and return safely.¢80 CLEANUP PLA ; Two bytes per parameter.¢90 PLA¢100 DEX ; Get all the parameters off?¢110 BNE CLEANUP ; when all gone, just the valid return addr¢120 ERROR RTS ; is at the top of the stack for the RTS.¢130 ; We have valid input, compute the sum.¢140 ; The first parameter in the USR call (after the addr)¢150 ; is the first parameter off the stack, high-byte¢160 ; low-byte sequence. REMEMBER this!¢170 AOK PLA ; GetNUM1, high byte¢180 STA NUM1+1¢190 PLA ; Get NUM1, low byte¢200 STA NUM1¢210 PLA ; Get NUM2, high byte¢220 STA NUM2+1¢230 PLA ; Get NUM2, low byte¢240 STA NUM2¢250 ; Now we have the data in temporary storage¢260 ; and the stack is cleared of parameters.¢270 ; Just the return address (to get us back to BASIC)¢280 ; is at the top of the stack, which gets pulled off¢290 ; into the program counter automatically by the RTS¢300 ; instruction.¢310 CLC ; Must clear the carry flag first.¢320 LDA NUM1 ; Low byte of first integer to add.¢330 ADC NUM2 ; Add to low byte of second integer.¢340 STA SUM ; And store in low byte of their SUM.¢350 LDA NUM1+1 ; Now add high bytes, leave carry alone.¢360 ADC NUM2+1 ; It "carries over" from previous add.¢370 STA SUM+1 ; And their summation is complete.¢380 RTS ; Back to BASIC¢
