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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Additional info for 6502 Assembly Language Subroutines
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.