ST20-GP1 datasheet(18/116 Pages) STMICROELECTRONICS

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ST20-GP1

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Central processing unit

The Central Processing Unit (CPU) is the ST20 32-bit processor core. It contains instruction

processing logic, instruction and data pointers, and an operand register. It can directly access the

high speed on-chip memory, which can store data or programs. Where larger amounts of memory

are required, the processor can access memory via the External Memory Interface (EMI).

The processor provides high performance:

•

Fast integer multiply — 3 cycle multiply

•

Fast bit shift — single cycle barrel shifter

•

Byte and part-word handling

•

Scheduling and interrupt support

•

64-bit integer arithmetic support

The scheduler provides a single level of pre-emption. In addition, multi-level pre-emption is

provided by the interrupt subsystem, see Chapter 5 for details. Additionally, there is a per-priority

trap handler to improve the support for arithmetic errors and illegal instructions, refer to section 4.6.

4.1

Registers

The CPU contains six registers which are used in the execution of a sequential integer process.

The six registers are:

•

The workspace pointer (Wptr) which points to an area of store where local data is kept.

•

The instruction pointer (IptrReg) which points to the next instruction to be executed.

•

The status register (StatusReg).

•

The Areg, Breg and Creg registers which form an evaluation stack.

The Areg, Breg and Creg registers are the sources and destinations for most arithmetic and

logical operations. Loading a value into the stack pushes Breg into Creg, and Areg into Breg,

before loading Areg. Storing a value from Areg, pops Breg into Areg and Creg into Breg. Creg is

left undeﬁned.

Figure 4.1 Registers used in sequential integer processes

Areg

Breg

Creg

Wptr

IptrReg

Local data

Program

Registers

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