AN910 datasheet(35/51 Pages) STMICROELECTRONICS

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ST7 AND ST9 PERFORMANCE BENCHMARKING

7.3.8 68HC08 MCU core

On-chip/Off-chip buses

- on-chip buses

16-bit address bus

8-bit data/program memory bus

- off-chip buses

8/16-bit address bus

up to 22-bit with memory expansion module

8-bit data/program memory bus

the two buses are multiplexed with ports

Memory Space

Von Neumann organization

- linear data/program memory space

64 kbytes

up to 4 Mbytes with memory expansion module

256-byte zero page

58-byte peripheral register space

direct addressable

upper 256-byte interrupt vector table = 128 interrupts

Arithmetic Logic Unit

8-bit datapath

- 8-bit operations

- special functions

8x8 unsigned multiplication

5 cycles

16/8 unsigned integer division

7 cycles

Instruction processing

Prefetch mechanism

- 1-byte queue

opcode lookahead register

Cpu internal buses

16-bit address bus, 8-bit data bus

(to be confirmed)

Instruction set

Cisc encoding

- CPI

1 cycle to 9 cycles

- average CPI

between 4 and 5 cycles

- IL

1 byte to 4 bytes

- average IL

between 2 and 3 bytes

- special addressing modes

indexed with 8-bit offset and post-increment

stack pointer relative (8/16-bit offset)

- special instructions

compare & branch like

decrement & branch like

memory-to-memory moves

direct to direct

direct

↔ indexed with post-increment

Programming model

Accumulator

- one 8-bit accumulator

- one 16-bit index register

- other registers

16-bit program counter

16-bit stack pointer

condition code register

+ / x

68HC08 CPU

LDA

#8, X

ADD #A0

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