ST20-C1 datasheet(22/205 Pages) STMICROELECTRONICS

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3.3 Registers

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global interrupt enable and timeslice enable, which are global and carried from one

process to another across a context switch.

• The mac_count, mac_buffer, mac_scale and mac_mode fields are used by

the multiply-accumulate instructions to hold initialization data which must be

saved when an exception occurs. See Chapter 5 for details of multiply accu-

mulation.

• The global_interrupt_enable bit enables external interrupts. Interrupts

remain enabled or disabled until explicitly disabled or enabled again. This bit is

global and is maintained when a process is descheduled.

• The local_interrupt_enable enables external interrupts. Clearing this bit dis-

ables external interrupts until the current process is descheduled. This is

needed when a process delegates part of its processing to a peripheral and

then deschedules until completion, as described in section 4.11.3.

• Overflo w, underflow and carry bits relating to arithmetic state are kept in the

status word.

The ST20-C1 maintains “sticky” bits in the status word which indicate whether

an overflow or underflow has occurred. This allows a complete expression to

be evaluated before testing whether an overflow has occurred. Overflow and

underflo w are chosen as they apply both to addition as well as multiply as

opposed to a more traditional method of replicating two bits out of the carry

Bit numbers

Full name

Meaning when set or meaning of value

0- 7

mac_count

Multiply-accumulate number of steps.

8- 10

mac_buffer

Multiply-accumulate data buffer size code.

11 - 12

mac_scale

Multiply-accumulate scaling code.

mac_mode

Multiply-accumulate accumulator format code.

global_interrupt_enable

Enable external interrupts until explicitly disabled.

local_interrupt_enable

Enable external interrupts. Clearing this bit disables inter-

rupts until the current process is 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 or trapped.

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.

26 - 31

timeslice_count

Timeslice counter.

Table 3.2 Status register bits

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