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otherwise. The

ErrorFlags may also be set by the seterr instruction or tested and

cleared by the

testerr instruction. The stoperr instruction stops the current process if

the

ErrorFlag is set. The low priority ErrorFlag is copied to the high priority when the

processor switches from low to high priority. The

HaltOnError flag ma y be set by the

sethalterr instruction, cleared by clrhalterr and tested by testhalterr.

3.2

The process descriptor and its associated register fields

In order to identify a process completely it is necessary to know: its workspace

address (in which the byte selector is always 0), and its priority (high or low). This

information is contained in the process descriptor. The workspace address of the

currently executing process is held in the workspace pointer register (Wptr) and the

priority is held in the flag Priority.

Wptr points to the current process workspace, which is always word-aligned. Priority

is the priority of the currently executing process where the value 1 indicates low

priority and 0 indicates high priority.

The process descriptor is formed from a pointer to the process workspace or-ed with

the priority flag at bit 0. Bit 1 is always set to 0.

Wdesc is defined so that the following invariants are obeyed:

Wptr = Wdesc

∧ WordSelectMask

Priority = Wdesc

∧ 1

Figure 3.1 Constituents of a process descriptor

Workspace address

Priority

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