By Lance A. Leventhal

ISBN-10: 0931988594

ISBN-13: 9780931988592

OOsborne/McGraw-Hill 1982. Wrap fresh with area put on. Binding tight, publication sq.. most sensible of ebook has a few soiling. Pages are fresh with out names, marks or highlights. Proceeds profit the Oro Valley Library.

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Extra info for 6502 Assembly Language Subroutines

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The extra offset of 1 is necessary because the top of the stack is empty. • The calling program must place the parameters in the stack before calling the subroutine. • Dynamically allocating space on the stack is difficult at best. If you wish to reduce the stack pointer by NRESLT, two general approaches are TSX ;MOVE STACK POINTER TO A VIA X TXA SEC SBC TAX TXS ;SUBTRACT NRESLT FROM POINTER ;RETURN STACK #NRESLT DIFFERENCE TO POINTER CHAPTER 1: GENERAL PROGRAMMING METHODS 47 or PUSHB LDX PHA #NRESLT ;COUNT = NRESLT ;MOVE STACK POINTER DOWN 1 DEX BNE PUSHB Either approach leaves NRESLT empty locations at the top of the stack as shown in Figure 1-5.

The Compare CHAPTER 1: GENERAL PROGRAMMING METHODS 23 instruction (CMP) is more useful than the Subtract instruction (SBC) because Compare does not change the accumulator or involve the Carry. Examples 1. Branch to DEST if the accumulator contains the number VALUE. CMP #VALUE ;IS DATA = VALUE? BEQ DEST ;YES, BRANCH We could also use index register X with CPX or index register Y with CPY. 2. Branch to DEST if the contents of the accumulator are not equal to the con tents of memory location ADDR.

Place the retrieved address in memory locations POINTL and POINTH. LDY LDA STA = ZERO #0 ; INDEX (PGZRO) ,Y ;GET LSB OF ADDRESS ;GET MSB OF ADDRESS POINTL 1NY LDA STA (PGZRO) ,Y POINTH This procedure allows you to move from one element to another in a linked list. 2. Retrieve data from the address currently in memory locations PGZRO and PGZRO-hi and increment that address by 1. INC #0 (PGZRO),Y PGZRO BNE DONE INC PGZRO+1 LDY LDA DONE ;INDEX ;GET = ZERO DATA USING POINTER POINTER BY ;UPDATE 1 NOP This procedure allows you to use the address in memory as a pointer to the next available location in a buffer.

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6502 Assembly Language Subroutines by Lance A. Leventhal

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