ST20-GP6 datasheet(19/123 Pages) STMICROELECTRONICS

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ST20-GP6

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Central processing unit

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

cessing 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 - 4 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 pro-

vided 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 (

Iptr) which points to the next instruction to be executed.

•

The status register (

Status).

•

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 logi-

cal operations. Loading a value into the stack pushes

Breg into Creg, and Areg into Breg, before

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

Iptr

Local data

Program

Registers

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