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By Allan E. Moose, Marian J. Lorenz

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Reading a byte of data in memory is, to the CPU , the same operation as reading a byte of data sent out from a peripheral device, such as the keyboard. Likewise writing or sending, a byte of data to memory or a peripheral are equivalent operations. Both the CPU and the peripherals or memory have to agree on whether a given data transfer is to be a read or a write operation. Therefore , there is a control bus that carries read/ write and other control signals from the CPU to external devices and memory.

Which page is chosen for a particular use depends in part on the design of the 6502 microprocessor and in part on choices made by the Atari's designers. Two pages are especially important to the 6502. These are page zero and page one. Page zero is important because it can be accessed by the processor faster and more easily than any other page. Page one is important because the 6502 uses it as the stack. Box 2 gives a summary of memory allocation. A more detailed memory map is given in Appendix D.

On power-up or after a reset, the stack begins at address 511, which in Hi-Byte / Lo-Byte form is 01,255. The stack pointer will, at this time, contain the byte 255, as data is stored or removed on the stack, the stack pointer is incremented or decremented so that it always gives the address of the next available stack location. 5. The Program Counter: Machine language instructions are stored in memory in order by address. The program counter insures that instructions are performed in the proper sequence .

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