IA6805E2
29 August 2007
Microprocessor Unit
As of Production Version 00
A(Accumulator):
The accumulator is an 8-bit register used to hold operands and results of arithmetic
calculations or data manipulations.
X(Index Register):
The index register is an 8-bit register used during the indexed addressing mode. It contains
an 8-bit value used to create an effective address. The index register may also be used as a
temporary storage area when not performing addressing operations.
PC(Program Counter):
The program counter is a 13-bit register that holds the address of the next instruction to be
performed by the MPU.
SP(Stack Pointer):
The stack pointer is a 13-bit register that holds the address of the next free location on the
stack. During an MPU reset or the reset stack pointer (RSP) instruction, the stack pointer is
set to location $007f. The seven most significant bits of the stack pointer are permanently
set to 0000001. They are appended to the six least significant register bits to produce an
address range down to location $0040. The stack pointer gets decremented as data is pushed
onto the stack and incremented as data is removed from the stack. The stack area of RAM is
used to store the return address on subroutine calls and the machine state during interrupts.
The maximum number of locations for the stack pointer is 64 bytes. If the stack goes
beyond this limit the stack pointer wraps around and points to its upper limit thereby losing
the previously stored information. Subroutine calls use 2 bytes of RAM on the stack and
interrupts use 5 bytes.
CC(Condition code Register):
The condition code register is a 5-bit register that indicates the results of the instruction just
executed. The bit is set if it is high. A program can individually test these bits and specific
actions can be taken as a result of their states. Following is an explanation of each bit.
C(Carry Bit):
The carry bit indicates that a carry or borrow out of the Arithmetic Logical Unit (ALU)
occurred during the last arithmetic instruction. This bit is also modified during bit test, shift,
rotate, and branch types of instructions.
Z(Zero Bit):
The zero bit indicates the result of the last arithmetic, logical, or data manipulation was zero.
N(Negative Bit):
The negative bit indicates the result to the last arithmetic, logical, or data manipulation was
negative (bit 7 in the result is high).
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