HCS12 datasheet(406/452 Pages) FREESCALE

406 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

406

Freescale Semiconductor

Instruction set changes offset each other to a certain extent. Programming

style also affects the rate at which instructions appear. As a test, the

BUFFALO monitor, an 8-Kbyte M68HC11 assembly code program, was

reassembled for the CPU12. The resulting object code is six bytes smaller

than the M68HC11 code. It is fair to conclude that M68HC11 code can be

reassembled with very little change in size.

The relative size of code for M68HC11 vs. code for CPU12 has also been

tested by rewriting several smaller programs from scratch. In these cases,

the CPU12 code is typically about 30 percent smaller. These savings are

mostly due to improved indexed addressing.

It seems useful to mention the results of size comparisons done on C

programs. A C program compiled for the CPU12 is about 30 percent smaller

than the same program compiled for the M68HC11. The savings are largely

due to better indexing.

Table B-2. Instructions with Smaller Object Code

Instruction

Comments

DEY

INY

Page 2 opcodes in M68HC11 but page 1 in CPU12

INST n,Y

For values of n less than 16 (the majority of cases). Were on page 2,

now are on page 1. Applies to BSET, BCLR, BRSET, BRCLR, NEG,

COM, LSR, ROR, ASR, ASL, ROL, DEC, INC, TST, JMP, CLR,

SUB, CMP, SBC, SUBD, ADDD, AND, BIT, LDA, STA, EOR, ADC,

ORA, ADD, JSR, LDS, and STS. If X is the index reference and the

offset is greater than 15 (much less frequent than offsets of 0, 1, and

2), the CPU12 instruction assembles to one byte more of object code

than the equivalent M68HC11 instruction.

PSHY

PULY

Were on page 2, now are on page 1

LDY

STY

CPY

Were on page 2, now are on page 1

CPY n,Y

LDY n,Y

STY n,Y

For values of n less than 16 (the majority of cases); were on page 3,

now are on page 1

CPD

Was on page 2, 3, or 4, now on page 1. In the case of indexed with

offset greater than 15, CPU12 and M68HC11 object code are the

same size.

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