Programmer’s Model

2.4Instruction length

Instructions are one of:

•32 bits long, in ARM state

•16 bits long, in Thumb state

•variable length, multiples of 8 bits, in Jazelle state.

Programmer’s Model

2.5Data types

The processor supports the following data types:

•word, 32-bit

•halfword, 16-bit

•byte, 8-bit.

Note

•When any of these types are described as unsigned, the N-bit data value represents a non-negative integer in the range 0 to +2N-1, using normal binary format.

•When any of these types are described as signed, the N-bit data value represents an integer in the range -2N-1 to +2N-1-1, using two’s complement format.

For best performance you must align these as follows:

•word quantities must be aligned to four-byte boundaries

•halfword quantities must be aligned to two-byte boundaries

•byte quantities can be placed on any byte boundary.

The processor provides mixed-endian and unaligned access support. For details see Chapter 4

Unaligned and Mixed-endian Data Access Support.

Note

You cannot use LDRD, LDM, LDC, STRD, STM, or STC instructions to access 32-bit quantities if they are unaligned.