AN910 datasheet(30/51 Pages) STMICROELECTRONICS

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ST7 AND ST9 PERFORMANCE BENCHMARKING

7.3.3 68HC12 MCU core

Memory Spaces

Von Neumann organization

- linear data/program memory space

64 kbytes with first 256-byte zero page

peripheral registers in zero page

upper 128-byte interrupt vector table = 64 interrupts

- memory extension (Harvard organization)

program/data/extra mem. windows in linear space

up to 4-Mbyte memory space/window

On-chip/Off-chip buses

- on-chip buses

16-bit address bus

8/16-bit data/program memory bus

- off-chip buses

16-bit address bus

up to 22 bits with memory expansion module

8/16-bit data/program memory bus

the two buses are multiplexed with ports

Arithmetic Logic Unit

20-bit datapath

- 8/16-bit operations

- special functions

8x8 unsigned multiplication

3 cycles

16x16 (un)signed multiplications

3 cycles

32/16 (un)signed divisions

(11)12 cycles

16/16 unsigned fractional division

12 cycles

16/16 (un)signed integer divisions

12 cycles

min/max of two 16-bit values

4 to 7 cycles

mac signed 16x16 to 32-bit mem.

13 cycles

8/16-bit table lookup and interpolate 10 cycles

(un)weighted product sum

8n cycles

Instruction processing

Prefetch mechanism

- 2-stage queue

2-word instruction queue

16-bit holding buffer if queue is full

- predecoding

- word operand even/odd alignment

no performance penalty if odd alignment

Cpu internal buses

16-bit address bus, 16-bit data bus

(to be confirmed)

Instruction set

Cisc encoding

- CPI

1 cycle to 13 cycles

- average CPI

between 3 and 4 cycles

- IL

1 byte to 5 bytes

- average IL

between 3 and 4 bytes

- special addressing modes

auto pre/post-increment/decrement indexed

stack pointer and program counter indexed

indexed-indirect with 16-bit offset

accumulator offset indexed

- special instructions

exchange register contents

increment/decrement/test & branch like

memory-to-memory moves

extended

↔ extended

mac & min/max instructions

fuzzy logic support, table lookup and interpolate

+ / x

Programming model

Accumulators

- two 8-bit accumulators

useable as one 16-bit accumulator

- two 16-bit index registers

- others registers

16-bit program counter

16-bit stack pointer

condition code register

- multitasking capabilities

with memory expansion module

context switching with program page register

and program/data/extra windows

specific call and rtc instructions

68HC12 CPU

LDAA

#8, X

ADDA #A0

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