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ST20C2

Instruction Set Reference Manual

STMicroelectronics 

This section also describes any situations where the operation of the instruction is

undefined or invalid.

Example

Using the add instruction as an example again, the comments listed are:

Comments:

Secondary instruction.

This says that add is a secondar y instruction.

1.7 Notation

The following sections give a full description of the notation used in the ‘definition’

section of the instruction descriptions.

1.7.1 The processor state

The processor state consists of the registers (mainly Areg, Breg, Creg, Iptr, and

Wptr), the contents of memory, and various flags and special registers (such as the

error flags , process queue pointers, clock registers, etc.).

The Wptr register is used for the address of the workspace of the current process.

This address is word aligned and therefore has the two least significant bits set to

zero. Wdesc is used for the ‘process descriptor’ — the value that is held in memory as

an identifier of the process when the process is not being executed. This value is

composed of the top 31 bits of the Wptr plus the process priority stored in bit 0 of the

word. Bit 0 is set to 0 for high priority processes and 1 for low priority processes. Bit 1

of the process descriptor is always 0.

1.7.2 General

The instruction descriptions are not intended to describe the way the instructions are

implemented, but only their effect on the state of the processor. So, for example, the

block move instructions are described in terms of a sequence of byte reads and writes

even though the instructions are implemented to perform the minimum number of

word reads and writes.

Comments (in italics) are used to both clarify the description and to describe actions

or values that cannot easily be represented by the notation used here; e.g. start next

process. These actions may be performed in another subsystem in the device, such

as the communications subsystem, and so any changes to machine state are not

necessarily completely synchronized with the execution of the instruction (as the

different subsystems work independently and in parallel).

Ellipses are used to show a range of values; e.g. ‘i = 0..31’ means that i has

values from 0 to 31, inclusive.

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