ARM ARM7TDMI User Manual Page 20

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Introduction
1-4 Copyright © 2001 ARM Limited. All rights reserved. ARM DDI 0234B
1.2 ARM7TDMI-S architecture
The ARM7TDMI-S processor has two instruction sets:
the 32-bit ARM instruction set
the 16-bit Thumb instruction set.
The ARM7TDMI-S processor is an implementation of the ARM architecture v4T. For
full details of both the ARM and Thumb instruction sets, see the ARM Architecture
Reference Manual.
1.2.1 Instruction compression
Microprocessor architectures traditionally had the same width for instructions and data.
Therefore, 32-bit architectures had higher performance manipulating 32-bit data and
could address a large address space much more efficiently than 16-bit architectures.
16-bit architectures typically had higher code density than 32-bit architectures, and
greater than half the performance.
Thumb implements a 16-bit instruction set on a 32-bit architecture to provide:
higher performance than a 16-bit architecture
higher code density than a 32-bit architecture.
1.2.2 The Thumb instruction set
The Thumb instruction set is a subset of the most commonly used 32-bit ARM
instructions. Thumb instructions are each 16 bits long, and have a corresponding 32-bit
ARM instruction that has the same effect on the processor model. Thumb instructions
operate with the standard ARM register configuration, allowing excellent
interoperability between ARM and Thumb states.
On execution, 16-bit Thumb instructions are transparently decompressed to full 32-bit
ARM instructions in real time, without performance loss.
Thumb has all the advantages of a 32-bit core:
32-bit address space
32-bit registers
32-bit shifter and Arithmetic Logic Unit (ALU)
32-bit memory transfer.
Thumb therefore offers a long branch range, powerful arithmetic operations, and a large
address space.
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