MC68HC705J2 Datasheet, PDF (24/82 Pages) Freescale Semiconductor, Inc – member of the low-cost

Freescale Semiconductor, Inc.
4.1.1 Accumulator
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The accumulator is a general-purpose 8-bit register. The CPU uses the
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accumulator to hold operands and results of arithmetic and nonarithmetic
operations.
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4.1.2 Index Register
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The 8-bit index register can perform two functions:
• Indexed addressing
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• Temporary storage
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In indexed addressing, the CPU uses the byte in the index register to determine
the conditional address of the operand.
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The index register can also serve as an auxiliary accumulator for temporary
storage.
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4.1.3 Stack Pointer
The stack pointer is a 16-bit register that contains the address of the next free
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location on the stack. During a reset or after the reset stack pointer (RSP)
instruction, the stack pointer contents are preset to $00FF. The address in the
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stack pointer decrements as data is pushed onto the stack and increments as data
is pulled from the stack.
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The ten most significant bits of the stack pointer are permanently fixed at
0000000011, so the stack pointer produces addresses from $00C0 to $00FF. If
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subroutines and interrupts use more than 64 stack locations, the stack pointer
wraps around to address $00C0 and begins writing over the previously stored data.
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A subroutine uses two stack locations; an interrupt uses five locations.
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CENTRAL PROCESSOR UNIT
4-2
Rev. 2
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