IA6805E2

29 August 2007

Microprocessor Unit

As of Production Version 00

Copyright © 2007

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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):

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).