Changes
6502
,| PC (Program Counter) || 16-bit || Points to the next instruction to be executed || Automatically increments as instructions are executed
|}
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== Memory Access ==
The address space that the 6502 uses is split into pages. There are 256 pages and each page is 256 bytes in size, ranging from page 0 to page 255.
In order to make up for the lack of registers, the 6502 includes a zero page addressing mode ($0000-$00FF) that uses only 1 address byte in the instruction instead of the 2 that are needed to address the full 64 KB of memory. This provides fast access to the first 256 bytes of RAM by using shorter instructions.
The stack is permanently located in page 1 ($0100-$01FF) and managed by the 8-bit stack pointer (S), with an initial value of $FF. It grows downward as data is pushed onto the stack. The stack has a 256-byte limit, and overflow occurs if not managed properly.
Instructions PHA and PHP push the accumulator and processor status onto the stack, while PLA and PLP pull them back. Subroutine calls with JSR store the return address on the stack, and RTS retrieves it to continue execution. Similarly, interrupts (BRK) push the program counter and status, while RTI restores them.
All I/O operations are memory-mapped. There are no port-based I/O instructions.
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On NMOS, DF is unchanged when entering an interrupt of any kind. This can cause unexpected bugs in the interrupt handler if Decimal Mode is on when an interrupt occurs. On CMOS, DF is automatically cleared on interrupt. Upon returning from an interrupt, the processor restores the status register from the stack, including DF.
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