6502 Assembly Language Subroutines by Lance A. Leventhal

By Lance A. Leventhal

OOsborne/McGraw-Hill 1982. Wrap fresh with aspect put on. Binding tight, e-book sq.. best of publication has a few soiling. Pages are fresh without names, marks or highlights. Proceeds gain the Oro Valley Library.

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3. Branch to DEST if the contents of memory address OPERl are less than or equal to the contents of memory address OPER2. LDA 0PER2 CMP OPERl ;GET SECOND OPERAND ;IS BCS DEST ;YES, IT GREATER THAN OR EQUAL TO FIRST? BRANCH If we loaded the accumulator with OPERl and compared to OPER2, we could branch only on the conditions • OPERl greater than or equal to OPER2 (Carry set) • OPERl less than OPER2 (Carry cleared) Since neither of these is what we want, we must handle the operands in the opposite order.

LDA CMP NOBOR EXIT #LOWL ;STACK POINTER AT OR BELOW LOW? SPTR LDA #LOWM SBC SPTR+1 BCS EXIT ;YES, LDA SPTR ;NO, BNE NOBOR DEC SPTR+1 DEC SPTR LDY #0 LDA NOP (SPTR),Y SET CARRY AND DECREMENT STACK EXIT (UNDERFLOW) POINTER :LOAD ACCUMULATOR FROM STACK 44 6502 ASSEMBLY LANGUAGE SUBROUTINES PARAMETER PASSING TECHNIQUES The most common ways to pass parameters on the 6502 microprocessor are 1. In registers. Three 8-bit registers are available (A, X, and Y). This approach is adequate in simple cases but it lacks generality and can handle only a limited number of parameters.

LDA #%00100000 ;GET MASK AND $75D0 ;SET FLAG IF BIT 5 IS ZERO IF BIT 5 IS ZERO We can also use the sequence LDA #%00100000 ;GET MASK BIT $75D0 ;SET FLAG If the bit position of interest is number 6, we can use the single instruction BIT $75D0 to set the Overflow flag to its value. 4. Set the Zero flag if the data at the input port assigned to memory address 1700l6islB16. LDA CMP #$1B $170U 5. Load the Carry flag with the data from bit 7 of the input port assigned to memory address 33A5I6. ASL $33A5 Note that this instruction does not change the data in memory location 33A516 unless that location is latched and buffered.

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