Cortex-A8 Technical Reference Manual - ARM Information Center

2.3 ThumbEE instruction set
2.3.1 Instructions
2.3.2 Configuration
Programmers Model
ThumbEE is a variant of the Thumb-2 instruction set. It is designed as a target for dynamically
generated code. This is code compiled on the device either shortly before or during execution
from a portable bytecode or other intermediate or native representation. It is particularly suited
to languages that employ managed pointers or array types. ThumbEE provides increased code
density for the compiled binary compared with the compiled code for the ARM or Thumb-2
instruction set. ThumbEE introduces a new processor state, the ThumbEE state, indicated by
both the T bit and the J bit in the CPSR Register being set to 1.
See the ARM Architecture Reference Manual for information on the ARM, Thumb, and
ThumbEE instruction sets.
In ThumbEE state, the processor uses almost the same instruction set as Thumb-2 although
some instructions behave differently, and a few are removed, or added.
The key differences are:
• additional state changing instructions in both Thumb state and ThumbEE state
• new instructions to branch to handlers
• null pointer checking on loads and stores
• an additional instruction in ThumbEE state to check array bounds
• some other modifications to the load, store, and branch instructions.
ThumbEE instructions are accessible when the processor is in ThumbEE state.
Two registers provide ThumbEE configuration:
• ThumbEE Configuration Register. This contains a single bit, the ThumbEE configuration
control bit, XED.
• ThumbEE HandlerBase Register. This contains the base address for ThumbEE handlers.
A handler is a short, commonly executed, sequence of instructions. It is typically, but not
always, associated directly with one or more bytecodes or other intermediate language
elements.
ThumbEE Configuration Register
The purpose of the ThumbEE Configuration Register is to control access to the ThumbEE
HandlerBase Register.
The ThumbEE Configuration Register is:
• in CP14 register c0
• a 32-bit register, with access rights that depend on the current privilege:
— the result of an unprivileged write to the register is Undefined
— unprivileged reads, and privileged reads and writes, are permitted.
Figure 2-2 on page 2-5 shows the bit arrangement of the ThumbEE Configuration Register.
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