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C165H
Central Processor Unit
Data Sheet
71
2001-04-19
PRELMNARY
The IP register is implicitly updated by the CPU for branch instructions and after
instruction fetch operations.
-EXCERPT-
MULIP-Flag:
The MULIP-flag will be set to '1' by hardware upon the entrance into an
interrupt service routine, when a multiply or divide ALU operation was interrupted before
completion. Depending on the state of the MULIP bit, the hardware decides whether a
multiplication or division must be continued or not after the end of an interrupt service.
The MULIP bit is overwritten with the contents of the stacked MULIP-flag when the
return-from-interrupt-instruction (RETI) is executed. This normally means that the
MULIP-flag is cleared again after that.
Note:
The MULIP flag is a part of the task environment! When the interrupting service
routine does not return to the interrupted multiply/divide instruction (ie. in case of
a task scheduler that switches between independent tasks), the MULIP flag must
be saved as part of the task environment and must be updated accordingly for the
new task before this task is entered.
CPU Interrupt Status (IEN, ILVL)
The Interrupt Enable bit allows to globally enable (IEN=’1’) or disable (IEN=’0’) interrupts.
The four-bit Interrupt Level field (ILVL) specifies the priority of the current CPU activity.
The interrupt level is updated by hardware upon entry into an interrupt service routine,
but it can also be modified via software to prevent other interrupts from being
acknowledged. In case an interrupt level '15' has been assigned to the CPU, it has the
highest possible priority, and thus the current CPU operation cannot be interrupted
except by hardware traps or external non-maskable interrupts. For details please refer
to chapter “Interrupt and Trap Functions”.
After reset all interrupts are globally disabled, and the lowest priority (ILVL=0) is
assigned to the initial CPU activity.
The Instruction Pointer IP
This register determines the 16-bit intra-segment address of the currently fetched
instruction within the code segment selected by the CSP register. The IP register is not
mapped into the C165H's address space, and thus it is not directly accessable by the
programmer. The IP can, however, be modified indirectly via the stack by means of a