Atari assembly language programmer's guide by Allan E. Moose, Marian J. Lorenz

By Allan E. Moose, Marian J. Lorenz

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Five of these are 8 bit registers . The sixth is a 16 bit register. The five 8 bit registers are the: 1. Accumulator 2. X-Register 3. V-Register 4. Processor Status Register 5. Stack Pointer. The remaining register is the 16 bit program counter. The functions of the registers are as follows: 1. The Accumulator: This is the busiest register in the CPU. It is the only register where arithmetic and logic operations can be performed. Data transfer from one memory location to another usually goes through the accumulator.

The form used is controlled by the two instructions: SED Set Decimal mode CLD Clear Decimal mode and Of course, SED causes the CPU to work in the decimal mode while CLD directs the CPU to act in the binary mode. Addition can be completed with or without a carry occuring as part of the result. Similarly subtraction can be performed with or without borrowing. Unlike some other processors, the 6502 only has instructions for addition with a carry and subtraction with borrow. The digit to be 'borrowed' is contributed by the carry flag of the status register.

Of these, the X and Y registers are most commonly used as counting or indexing registers to either keep track of how many times we've cycled through a routine or to successively locate items of data in a table. 2 OVERVIEW OF 6502 INSTRUCTIONS Introduction Table 2-1 is a list of the fifty-six different instruction names for the 6502 CPU. Each instruction name has been coded into a three letter mnemonic that is suggestive of the task to be carried out. Roughly half of these instructions perform simple workman-like jobs such as transferring the contents of one register into another, incrementing or decrementing a register, and setting or clearing a bit .

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