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

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By Allan E. Moose, Marian J. Lorenz

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3. The Status Register: This register contains seven usable bits . Two of the bits are control bits . The remaining five are status flags The status flags provide information on the result of a previously executed instruction (usually the preceding instruction). The 6502 can be programmed to test the condition of each of these flags . Based on the results of these tests, the 6502 can choose between two possible sequences of instructions . The locations , labels, and functions of these bits are described in Box 1.

The 6502 can add or subtract numb ers in a binary form or binary coded decimal form. 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.

3. POKEY: is a digital I / O chip that has a variety offunctions. One of the foremost is sound generation . It is the keeper of the registers that control the frequency and type of sound output. In addition POKEY takes care of transmission of data from the keyboard and from the serial communications port. Serial data transfer differs from the parallel data transfer mentioned earlier in that data is transmitted one bit at a time. Other functions of POKEY will be described in Chapter Six. 4. PIA: is the Peripheral Interface Adaptor chip with the primary responsibility for controlling data to and from the joystick ports .

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