ST20-C1 datasheet(170/205 Pages) STMICROELECTRONICS

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A.1

Status Register

A.2

Exception levels

Full name

Bit numbers

Meaning when set or meaning of value

mac_count

0 - 7

Multiply-accumulate number of steps.

mac_buffer

8 - 10

Multiply-accumulate data buffer size code.

mac_scale

11 - 12

Multiply-accumulate scaling code.

mac_mode

Multiply-accumulate accumulator format code.

global_interrupt_disable

Disable external interrupts until explicitly enabled.

local_interrupt_disable

Disable external interrupts until process descheduled.

overflo w

An arithmetic operation gave a positive overflow.

underflo w

An arithmetic operation gave a negative overflow.

carry

An arithmetic operation produced a carry.

user_mode

A user process is executing.

interrupt_mode

An interrupt handler is executing.

trap_mode

A trap handler is executing.

sleep

The processor is due to go to sleep.

reserved

Reserved.

start_next_task

The CPU must start executing a new process.

timeslice_enable

Timeslicing is enabled.

timeslice_count

26 - 31

Timeslice clock.

Table A.1 Status register bits (see section 3.3.2)

Exception level

Name

Description

0 - 255

User exception

User exception levels.

-1

el_breakpoint_trap

Breakpoint trap.

-2

el_illegal_instr_trap

Illegal op-code trap.

-3

el_idle_trap

CPU idle trap.

-4

el_schedule_exception_trap

Schedule user process as exception trap.

-5

el_run_trap

Run process trap.

-6

el_stop_trap

Stop process trap.

-7

el_timeslice_trap

Timeslice trap.

Table A.2 Exception levels (see Chapter 6)

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