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base:6502_registers [2015-04-17 04:30] (current)
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 +====== 6502 Registers ======
 +The NMOS 65xx processors are not ruined with too many registers. In addition to that, the registers are mostly 8-bit. Here is a brief description of each register:
 +       PC   Program Counter
 +            This register points the address from which the next
 +            instruction byte (opcode or parameter) will be fetched.
 +            Unlike other registers, this one is 16 bits in length. The
 +            low and high 8-bit halves of the register are called PCL
 +            and PCH, respectively.
 +            The Program Counter may be read by pushing its value on
 +            the stack. This can be done either by jumping to a
 +            subroutine or by causing an interrupt.
 +          Stack pointer
 +            The NMOS 65xx processors have 256 bytes of stack memory,
 +            ranging from $0100 to $01FF. The S register is a 8-bit
 +            offset to the stack page. In other words, whenever
 +            anything is being pushed on the stack, it will be stored
 +            to the address $0100+S.
 +            The Stack pointer can be read and written by transfering
 +            its value to or from the index register X (see below) with
 +            the TSX and TXS instructions.
 +          Processor status
 +            This 8-bit register stores the state of the processor. The
 +            bits in this register are called flags. Most of the flags
 +            have something to do with arithmetic operations.
 +            The P register can be read by pushing it on the stack
 +            (with PHP or by causing an interrupt). If you only need to
 +            read one flag, you can use the branch instructions.
 +            Setting the flags is possible by pulling the P register
 +            from stack or by using the flag set or clear instructions.
 +            Following is a list of the flags, starting from the 8th
 +            bit of the P register (bit 7, value $80):
 +            N   Negative flag
 +                This flag will be set after any arithmetic operations
 +                (when any of the registers A, X or Y is being loaded
 +                with a value). Generally, the N flag will be copied
 +                from the topmost bit of the register being loaded.
 +                Note that TXS (Transfer X to S) is not an arithmetic
 +                operation. Also note that the BIT instruction affects
 +                the Negative flag just like arithmetic operations.
 +                Finally, the Negative flag behaves differently in
 +                Decimal operations (see description below).
 +            V   oVerflow flag
 +                Like the Negative flag, this flag is intended to be
 +                used with 8-bit signed integer numbers. The flag will
 +                be affected by addition and subtraction, the
 +                instructions PLP, CLV and BIT, and the hardware signal
 +                -SO. Note that there is no SEV instruction, even though
 +                the MOS engineers loved to use East European abbreviations,
 +                like DDR (Deutsche Demokratische Republik vs. Data
 +                Direction Register). (The Russian abbreviation for their
 +                former trade association COMECON is SEV.) The -SO
 +                (Set Overflow) signal is available on some processors,
 +                at least the 6502, to set the V flag. This enables
 +                response to an I/O activity in equal or less than
 +                three clock cycles when using a BVC instruction branching
 +                to itself ($50 $FE).
 +                The CLV instruction clears the V flag, and the PLP and
 +                BIT instructions copy the flag value from the bit 6 of
 +                the topmost stack entry or from memory.
 +                After a binary addition or subtraction, the V flag
 +                will be set on a sign overflow, cleared otherwise.
 +                What is a sign overflow?  For instance, if you are
 +                trying to add 123 and 45 together, the result (168)
 +                does not fit in a 8-bit signed integer (upper limit
 +                127 and lower limit -128). Similarly, adding -123 to
 +                -45 causes the overflow, just like subtracting -45
 +                from 123 or 123 from -45 would do.
 +                Like the N flag, the V flag will not be set as
 +                expected in the Decimal mode. Later in this document
 +                is a precise operation description.
 +                A common misbelief is that the V flag could only be
 +                set by arithmetic operations, not cleared.
 +            1   unused flag
 +                To the current knowledge, this flag is always 1.
 +            B   Break flag
 +                This flag is used to distinguish software (BRK)
 +                interrupts from hardware interrupts (IRQ or NMI). The
 +                B flag is always set except when the P register is
 +                being pushed on stack when jumping to an interrupt
 +                routine to process only a hardware interrupt.
 +                The official NMOS 65xx documentation claims that the
 +                BRK instruction could only cause a jump to the IRQ
 +                vector ($FFFE). However, if an NMI interrupt occurs
 +                while executing a BRK instruction, the processor will
 +                jump to the NMI vector ($FFFA), and the P register
 +                will be pushed on the stack with the B flag set.
 +            D   Decimal mode flag
 +                This flag is used to select the (Binary Coded) Decimal
 +                mode for addition and subtraction. In most
 +                applications, the flag is zero.
 +                The Decimal mode has many oddities, and it operates
 +                differently on CMOS processors. See the description
 +                of the ADC, SBC and ARR instructions below.
 +            I   Interrupt disable flag
 +                This flag can be used to prevent the processor from
 +                jumping to the IRQ handler vector ($FFFE) whenever the
 +                hardware line -IRQ is active. The flag will be
 +                automatically set after taking an interrupt, so that
 +                the processor would not keep jumping to the interrupt
 +                routine if the -IRQ signal remains low for several
 +                clock cycles.
 +            Z   Zero flag
 +                The Zero flag will be affected in the same cases than
 +                the Negative flag. Generally, it will be set if an
 +                arithmetic register is being loaded with the value
 +                zero, and cleared otherwise. The flag will behave
 +                differently in Decimal operations.
 +            C   Carry flag
 +                This flag is used in additions, subtractions,
 +                comparisons and bit rotations. In additions and
 +                subtractions, it acts as a 9th bit and lets you to
 +                chain operations to calculate with bigger than 8-bit
 +                numbers. When subtracting, the Carry flag is the
 +                negative of Borrow: if an overflow occurs, the flag
 +                will be clear, otherwise set. Comparisons are a
 +                special case of subtraction: they assume Carry flag
 +                set and Decimal flag clear, and do not store the
 +                result of the subtraction anywhere.
 +                There are four kinds of bit rotations. All of them
 +                store the bit that is being rotated off to the Carry
 +                flag. The left shifting instructions are ROL and ASL.
 +                ROL copies the initial Carry flag to the lowmost bit
 +                of the byte; ASL always clears it. Similarly, the ROR
 +                and LSR instructions shift to the right.
 +          Accumulator
 +            The accumulator is the main register for arithmetic and
 +            logic operations. Unlike the index registers X and Y, it
 +            has a direct connection to the Arithmetic and Logic Unit
 +            (ALU). This is why many operations are only available for
 +            the accumulator, not the index registers.
 +          Index register X
 +            This is the main register for addressing data with
 +            indices. It has a special addressing mode, indexed
 +            indirect, which lets you to have a vector table on the
 +            zero page.
 +          Index register Y
 +            The Y register has the least operations available. On the
 +            other hand, only it has the indirect indexed addressing
 +            mode that enables access to any memory place without
 +            having to use self-modifying code.
base/6502_registers.txt ยท Last modified: 2015-04-17 04:30 (external edit)