Except for a single accumulator ( called W ), almost all other registers are memory-mapped, even registers like the program counter and ALU status register . ( The other exceptions, which are not memory-mapped, are the return address stack, and the tri-state registers used to configure the GPIO pins .)
42.
The processor contains 16 16-bit registers, of which 4 are dedicated to special purposes : R0 is the program counter, R1 is the stack pointer, R2 is the status register, and R3 is a special register called the " constant generator ", providing access to 6 commonly used constant values without requiring an additional operand.
43.
A part of the preservation process is automatically handled by the MPU when it acknowledges the interrupt, as it will push the program counter ( and program bank in the 65C816 / 65C802 ) and status register to the stack prior to executing the ISR . At the completion of the ISR, when the instruction is executed, the MPU will reverse the process.
44.
In computer processors, the "'overflow flag "'( sometimes called V flag ) is usually a single bit in a system status register used to indicate when an arithmetic overflow has occurred in an operation, indicating that the signed two's-complement result would not fit in the number of bits used for the operation ( the ALU width ).
45.
Additionally, with the 65 ( c ) 02 or the 65C816 / 65C802 operating in emulation mode, the copy of the status register that is pushed on to the stack will have the flag set if a ( software interrupt ) was the cause of the interrupt, or cleared if an was the cause . flag in the status register itself is always, regardless of the interrupt type . is meaningful only in the copy of the status register that is pushed onto the stack in response to an interrupt.
46.
Additionally, with the 65 ( c ) 02 or the 65C816 / 65C802 operating in emulation mode, the copy of the status register that is pushed on to the stack will have the flag set if a ( software interrupt ) was the cause of the interrupt, or cleared if an was the cause . flag in the status register itself is always, regardless of the interrupt type . is meaningful only in the copy of the status register that is pushed onto the stack in response to an interrupt.
47.
Additionally, with the 65 ( c ) 02 or the 65C816 / 65C802 operating in emulation mode, the copy of the status register that is pushed on to the stack will have the flag set if a ( software interrupt ) was the cause of the interrupt, or cleared if an was the cause . flag in the status register itself is always, regardless of the interrupt type . is meaningful only in the copy of the status register that is pushed onto the stack in response to an interrupt.
48.
The first 32 bytes of the register space are allocated to special-purpose registers; the remaining 96 bytes are used for general-purpose RAM . If banked RAM is used, the high 16 registers ( 0x70 0x7F ) are global, as are a few of the most important special-purpose registers, including the STATUS register which holds the RAM bank select bits . ( The other global registers are FSR and INDF, the low 8 bits of the program counter PCL, the PC high preload register PCLATH, and the master interrupt control register INTCON .)
