Epson ARM720T 8

8: Coprocessor Interface

ARM720T CORE CPU MANUAL EPSON 8-1

8Coprocessor Interface

This chapter describes the coprocessor interface on the ARM720T processor. It contains the

following sections:

8.1 About coprocessors ..................................................................................... 8-1

8.2 Coprocessor interface signals .................................................................... 8-3

8.3 Pipeline-following signals .......................................................................... 8-4

8.4 Coprocessor interface handshaking ......................................... ................. 8-5

8.5 Connecting coprocessors ............................................................................ 8-9

8.6 Not using an external coprocessor.................................... ....................... 8-10

8.7 STC operations ......................................................................................... 8-10

8.8 Undefined instructions ............................................................................ 8-10

8.9 Privileged instructions ............................................................................. 8-10

The instruction set for the ARM720T processor enables you to implement specialized

additional instructions using coprocessors. These are separate processing units that are

tightly coupled to the ARM720T processor. A typical coprocessor contains:

• an instruction pipeline

• instruction decoding logic

• handshake logic

•a register bank

• special processing logic, with its own data path.

A coprocessor is connected to the same data bus as the ARM720T processor in the system, and

tracks the pipeline in the ARM720T core. This means that the coprocessor can decode the

instructions in the instruction stream, and execute those that it supports. Each instruction

progresses down both the ARM720T processor pipeline and the coprocessor pipeline at the

same time.

The execution of instructions is shared between the ARM720T core and the coprocessor, as

follows:

The ARM720T core

1 Evaluates the condition codes to determine whether the

instruction must be executed by the coprocessor, then signals

this to any coprocessors in the system (using CPnCPI).

2 Generates any addresses that are required by the instruction,

including prefetching the next instruction to refill the

pipeline.

3 Takes the undefined instruction trap if no coprocessor accepts

the instruction.