Datasheet

Introduction

ARM7TDMI Data Sheet

ARM DDI 0029E

1-3

Open Access

1.2.2 THUMB’s Advantages

THUMB instructions operate with the standard ARM register conﬁguration, allowing

excellent interoperability between ARM and THUMB states. Each 16-bit THUMB

instruction has a corresponding 32-bit ARM instruction with the same effect on the

processor model.

The major advantage of a 32-bit (ARM) architecture over a 16-bit architecture is its

ability to manipulate 32-bit integers with single instructions, and to address a large

address space efﬁciently. When processing 32-bit data, a 16-bit architecture will take

at least two instructions to perform the same task as a single ARM instruction.

However, not all the code in a program will process 32-bit data (for example, code that

performs character string handling), and some instructions, like Branches, do not

process any data at all.

If a 16-bit architecture only has 16-bit instructions, and a 32-bit architecture only has

32-bit instructions, then overall the 16-bit architecture will have better code density,

and better than one half the performance of the 32-bit architecture. Clearly 32-bit

performance comes at the cost of code density.

THUMB breaks this constraint by implementing a 16-bit instruction length on a 32-bit

architecture, making the processing of 32-bit data efﬁcient with a compact instruction

coding. This provides far better performance than a 16-bit architecture, with better

code density than a 32-bit architecture.

THUMB also has a major advantage over other 32-bit architectures with 16-bit

instructions. This is the ability to switch back to full ARM code and execute at full

speed. Thus critical loops for applications such as

• fast interrupts

• DSP algorithms

can be coded using the full ARM instruction set, and linked with THUMB code. The

overhead of switching from THUMB code to ARM code is folded into sub-routine entry

time. Various portions of a system can be optimised for speed or for code density by

switching between THUMB and ARM execution as appropriate.