Download e-book for kindle: 6502 Assembly Language Subroutines by Lance A. Leventhal

By Lance A. Leventhal

ISBN-10: 0931988594

ISBN-13: 9780931988592

OOsborne/McGraw-Hill 1982. Wrap fresh with aspect put on. Binding tight, ebook sq.. best of booklet has a few soiling. Pages are fresh with out names, marks or highlights. Proceeds gain the Oro Valley Library.

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Additional info for 6502 Assembly Language Subroutines

Example text

Examples 1. Shift the accumulator right 1 bit arithmetically, preserving the sign (most significant) bit. TAX ASL TXA A ROR A ;SAVE THE ACCUMULATOR ;MOVE BIT 7 TO CARRY ;RESTORE THE ACCUMULATOR ;SHIFT THE ACCUMULATOR, COPYING BIT 7 When the processor performs ROR A, it moves the Carry (the old bit 7) to bit 7 and bit 7 to bit 6, thus preserving the sign of the original number. 2. Shift the accumulator left 1 bit arithmetically, preserving the sign (most sig nificant) bit. ASL A ROL A ;SHIFT A, MOVING BIT 7 TO CARRY ;SAVE BIT 7 IN POSITION 0 A ;CHANGE ROR A ;SHIFT THE ASL A CLRSGN #%10000000 ;SHIFT A, MOVING BIT ;WAS BIT 7 1?

Branch to DEST if bit 2 of the accumulator is 0. AND #%00000100 BEQ DEST ;TEST BIT 2 OF A There are special short procedures for examining bit positions 0, 6, or 7. Bit 7 is available readily as the Negative flag after a Load or Transfer instruction; bit 0 can be moved to the Carry with LSR A or ROR A; bit 6 can be moved to the Negative flag with ASL A or ROL A. 3. Branch to DEST if bit 7 of memory location ADDR is 1. LDA BMI ADDR DEST ;IS BIT 7 1? ;YESf BRANCH Note that LDA affects the Zero and Negative flags; so do transfer instructions such as TAX, TYA, TSX (but not TXS), and PLA.

Handling one character at a time is just like handling normal 8-bit unsigned numbers. Some examples are • Branch to address DEST if the accumulator contains an ASCII E. CMP #'E ;IS BEQ DEST ;YES, DATA E? BRANCH • Search a string starting at address STRNG until a non-blank character is found. EXAMC LDX LDA CMP #0 #■ STRNG,X ;POINT TO START OF STRING ;GET A BLANK FOR CHECKING ;IS NEXT CHARACTER A BLANK? DONE ;NO, DONE ;YES,PROCEED TO NEXT CHARACTER DONE BNE INX JMP NOP LDX LDA #$FF #' ;POINT TO BYTE BEFORE START ;GET A BLANK FOR COMPARISON INX CMP STRNG,X ;PROCEED TO NEXT ;IS IT A BLANK?

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6502 Assembly Language Subroutines by Lance A. Leventhal

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