Download PDF by Lance A. Leventhal: 6502 Assembly Language Subroutines

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 e-book has a few soiling. Pages are fresh without names, marks or highlights. Proceeds gain the Oro Valley Library.

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In real applications (see Chapter 10), the device table will probably contain the starting addresses of I/O subroutines (drivers) rather than actual device addresses. STATUS AND CONTROL You can handle status and control signals like any other data. The only special problem is that the processor may not be able to read output ports; in that case, you must retain copies (in RAM) of the data sent to those ports. Examples 1. Branch to address DEST if bit 3 of the input port assigned to memory address A100l6 is 1.

LDA STAT ;GET AND CMP *%0111U000 #%01010000 ;MASK OFF BITS 4, 5, ;IS STATUS FIELD 5? STATUS BEQ DEST ;YESf AND 6 BRANCH 3. Set bit 5 of address CNTL to 1. LDA CNTL ;GET ORA #%00100000 ;SET BIT CURRENT 5 DATA STA CNTL ;RESTORE DATA TO FROM PORT PORT If address CNTL cannot be read properly, we can use a copy in memory address TEMP. LDA ORA TEMP #%00100000 STA CNTL STA TEMP ;GET CURRENT ;SET BIT DATA FROM 5 ;RESTORE DATA TO PORT ;UPDATE COPY OF DATA PORT CHAPTER 1: GENERAL PROGRAMMING METHODS 53 You must update the copy every time you change the data.

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. CMP ADDR ;IS DATA = VALUE IN MEMORY? BNE DEST ;NO, BRANCH 3. Branch to DEST if memory location ADDR contains 0. LDA BEQ ADDR DEST ;IS DATA ZERO? ;YES, BRANCH We can handle some special cases without using the accumulator.

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6502 Assembly Language Subroutines by Lance A. Leventhal

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