ST20-C1 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'ST20-C1' PDF : 205 Pages View PDF
3.3 Registers
Bit numbers
Full name
0-7
mac_count
8 - 10 mac_buffer
11 - 12 mac_scale
13
mac_mode
14
global_interrupt_enable
15
local_interrupt_enable
16
17
18
19
20
21
22
23
24
25
26 - 31
overflo w
underflo w
carry
user_mode
interrupt_mode
trap_mode
sleep
reserved
start_next_task
timeslice_enable
timeslice_count
Meaning when set or meaning of value
Multiply-accumulate number of steps.
Multiply-accumulate data buffer size code.
Multiply-accumulate scaling code.
Multiply-accumulate accumulator format code.
Enable external interrupts until explicitly disabled.
Enable external interrupts. Clearing this bit disables inter-
rupts until the current process is descheduled.
An arithmetic operation gave a positive overflow.
An arithmetic operation gave a negative overflow.
An arithmetic operation produced a carry.
A user process is executing.
An interrupt handler is executing or trapped.
A trap handler is executing.
The processor is due to go to sleep.
Reserved.
The CPU must start executing a new process.
Timeslicing is enabled.
Timeslice counter.
Table 3.2 Status register bits
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 b y
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, underflo w 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
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