ARM Assembly Programming – LittlEndian Memory Addressing

Memory Addressing

ARMlite models

“byte” addressable ARMLITE memory

MEMORY

ADDRESSING That is

every byte can be indexed individually

BYTE ADDRESSABLE MEMORY

Address: 0x00000

BYTE ADDRESSABLE MEMORY

Address: 0x00001

BYTE ADDRESSABLE MEMORY

Address: 0x00002

BYTE ADDRESSABLE MEMORY

Address: 0x00003

BYTE ADDRESSABLE MEMORY

Address: 0x00004

BYTE ADDRESSABLE MEMORY

Address: 0x00005

BYTE ADDRESSABLE MEMORY

Address: 0x00006

BYTE ADDRESSABLE MEMORY

Address: 0x00007

BYTE ADDRESSABLE MEMORY

What I just showed you hopefully made complete sense

The problem is …..

What I just showed you hopefully made complete sense

The problem is …..

I lied

ARMLITE USES LITTLE ENDIAN

• Recall Big Endian versus Little Endian
• Big Endian:
• The most significant bits are represented first in memory
• Little Endian:
• The least significant bits are represented first in memory
• ARMlite displays memory values in Big Endian format
• BUT – addresses them using Little Endian

So …..

LITTLEENDIAN BYTE ADDRESSABLE MEMORY

Address: 0x00000

LITTLE ENDIAN BYTE ADDRESSABLE MEMORY

Address: 0x00001

LITTLE ENDIAN BYTE ADDRESSABLE MEMORY

Address: 0x00002

LITTLE ENDIAN BYTE ADDRESSABLE MEMORY

Address: 0x00003

LITTLE ENDIAN BYTE ADDRESSABLE MEMORY

Address: 0x00004

LITTLE ENDIAN BYTE ADDRESSABLE MEMORY

Address: 0x00005

LITTLE ENDIAN BYTE ADDRESSABLE MEMORY

Address: 0x00006

LITTLE ENDIAN BYTE ADDRESSABLE MEMORY

Address: 0x00007

SUMMARY

• When working at the level of bytes and bits, endianness matters a lot.
• In all our ARMlite programming, remember we will be working with bit strings in Little Endian format in memory (despite how they are displayed on the screen!)