ST20-GP1 datasheet(29/116 Pages) STMICROELECTRONICS

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ST20-GP1

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5.1

Interrupt vector table

The interrupt controller contains a table of pointers to interrupt handlers. Each interrupt handler is

represented by its workspace pointer (Wptr). The table contains a workspace pointer for each level

of interrupt.

The Wptr gives access to the code, data and interrupt save space of the interrupt handler. The

position of the Wptr in the interrupt table implies the priority of the interrupt.

Run-time library support is provided for setting and programming the vector table.

5.2

Interrupt handlers

At any interruptible point in its execution the CPU can receive an interrupt request from the

interrupt controller. The CPU immediately acknowledges the request.

In response to receiving an interrupt the CPU performs a procedure call to the process in the

vector table. The state of the interrupted process is stored in the workspace of the interrupt handler

as shown in Figure 5.2. Each interrupt level has its own workspace.

Figure 5.2 State of interrupted process

The interrupt routine is initialized with space below Wptr. The Iptr and Status word for the routine

are stored there permanently. This should be programmed before the Wptr is written into the vector

table. The behavior of the interrupt differs depending on the priority of the CPU when the interrupt

occurs.

When an interrupt occurs when the CPU was running at high priority, the CPU saves the current

process state (Areg, Breg, Creg, Wptr, Iptr and Status) into the workspace of the interrupt

handler. The value HandlerWptr, which is stored in the interrupt controller, points to the top of this

Handler Status

Before interrupt

Wptr

Handler Iptr

Areg

Breg

Creg

Interrupting high priority

Wptr

Iptr

Status

Wptr

Null Status

process

Interrupting low priority

process or CPU idle

Handler Status

Handler Iptr

Handler Status

Handler Iptr

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