ST20-GP1 datasheet(19/116 Pages) STMICROELECTRONICS

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ST20-GP1

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Expressions are evaluated on the evaluation stack, and instructions refer to the stack implicitly. For

example, the

add instruction adds the top two values in the stack and places the result on the top of

the stack. The use of a stack removes the need for instructions to explicitly specify the location of

their operands. No hardware mechanism is provided to detect that more than three values have

been loaded onto the stack; it is easy for the compiler to ensure that this never happens.

Note that a location in memory can be accessed relative to the workspace pointer, enabling the

workspace to be of any size.

The use of shadow registers provides fast, simple and clean context switching.

4.2

Processes and concurrency

The following section describes ‘default’ behavior of the CPU and it should be noted that the user

can alter this behavior, for example, by disabling timeslicing, installing a user scheduler, etc.

A process starts, performs a number of actions, and then either stops without completing or

terminates complete. Typically, a process is a sequence of instructions. The CPU can run several

processes in parallel (concurrently). Processes may be assigned either high or low priority, and

there may be any number of each.

The processor has a microcoded scheduler which enables any number of concurrent processes to

be executed together, sharing the processor time. This removes the need for a software kernel,

although kernels can still be written if desired.

At any time, a process may be

active

being executed

interrupted by a higher priority process

on a list waiting to be executed

inactive

waiting to input

waiting to output

waiting until a speciﬁed time

The scheduler operates in such a way that inactive processes do not consume any processor time.

Each active high priority process executes until it becomes inactive. The scheduler allocates a

portion of the processor’s time to each active low priority process in turn (see Section 4.3). Active

processes waiting to be executed are held in two linked lists of process workspaces, one of high

priority processes and one of low priority processes. Each list is implemented using two registers,

one of which points to the ﬁrst process in the list, the other to the last. In the linked process list

shown in Figure 4.2, process

S is executing and P, Q and R are active, awaiting execution. Only the

low priority process queue registers are shown; the high priority process ones behave in a similar

manner.

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