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Registers

3.1

Machine registers

This section introduces the ST20 processor registers that are visible to the

programmer. Firstly the set of registers known as state registers are presented and

discussed. These fully define the state of the executing process. Secondly the other

registers of interest to the programmer, are presented.

3.1.1

Process state registers

The state of a executing process at any instant is defined b y the contents of the

machine registers listed in Table 3.1. The ‘register’ column gives the abbreviated

name of the register. The ‘full name / description’ column provides the full textual

name which is usually used when referencing a register in this manual; and where

unclear, a brief description of the information contained in this register.

In addition there is a small number of registers used to implement block moves.

Status register

The Status register contains status bits which describe the current state of the

process and any errors which may have occurred. The contents of the Status register

are shown in Table 3.2.

‘Shadow’ registers

When a high priority process interrupts a low priority process, the state of the currently

executing process needs to be saved. For this purpose, two sets of process state

registers are provided, one each for high and low priority. On interrupt, the processor

switches to using the high priority registers, leaving the low priority registers to

preserve the low priority state.

A high priority process may manipulate the low priority ‘shadow’ registers with the

instructions

ldshadow and stshadow. In the definitions of these instructions, the

process state registers have a subscript (e.g. Areg[LowPriority]) indicating the priority.

full name / description process modes

Status

status register

Wptr

workspace pointer - contains the address of the workspace of the currently execut-

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