ARM11 BE8 and BE32

What’s difference between BE8 and BE32?

BE-32 is supported by ARM cores up to the ARM11 family (v6) (for example ARM7TDMI, ARM926EJ-S, ARM1136JF-S). It is enabled by setting a bit in the CP15 system control coprocessor.

BE-8 is supported by the ARM11 family and later (for example ARM1136JF-S, Cortex-R4, Cortex-A8). Architecture v7 cores do not support BE-32.

It is controlled by setting a bit in the CPSR.

Setting both bits is reserved (not a valid configuration).

In terms of data access:

BE-8 is byte invariant endianness

BE-32 is word invariant endianness

This is easiest to see with examples. I’ve starred **** the important ones:

Basic endianness:

Consider a word stored 0x11223344 where 11 is the most significant byte.

Little endian:

Address 0 1 2 3

Data 44 33 22 11

Big endian:

Address 0 1 2 3

Data 11 22 33 44

BE-32 and BE-8

Now consider data stored like this:

Address 0 1 2 3

Data 11 22 33 44

Core in little-endian mode makes word access to address 0:

LDR r0, [0]

r0 contains 0x44332211

Data loaded to register as little endian

Core in little-endian mode makes byte access to address 0:

LDRB r0, [0]

r0 contains 0x00000011

Data loaded from 0

Core in little-endian mode makes byte access to address 3:

LDRB r0, [3]

r0 contains 0x00000044

Data loaded from 3

Core in BE-32 mode makes word access to address 0:

LDR r0, [0]

r0 contains 0x44332211

Word accesses are endianness-invariant

****Core in BE-32 mode makes word access to address 0:

LDRB r0, [0]

r0 contains 0x00000044

Byte access in BE-32 reads the word as if it was stored big-endian

Core in BE-32 mode makes word access to address 3:

LDRB r0, [1]

r0 contains 0x00000011

As above

****Core in BE-8 mode makes word access to address 0:

LDR r0, [0]

r0 contains 0x11223344

Data loaded to register as big endian

Core in BE-8 mode makes byte access to address 0:

LDRB r0, [0]

r0 contains 0x00000011

Byte at address 0 is loaded, NOT the byte at address 3

Core in BE-8 mode makes byte access to address 3:

LDRB r0, [3]

r0 contains 0x00000044

Byte at address 3 is loaded.

Essentially BE-32 operates by altering the addresses of memory accesses when accessing subword quantities. This gives the appearances of big endian.

On this day..

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