Filter
Top Products
Altera Corporation 5
GettingOverview
Overview
1
Interrrupt Enable (IE)
IE is the interrupt enable bit. When IE=1, it enables external interrupts and
internal exceptions. IE=0 disables external interrupts and exceptions.
Software TRAP instructions will still execute normally even when IE=0.
Note that IE can be set directly without affecting the rest of the STATUS
register by writing to the SET_IE (%ctl9) and CLR_IE (%ctl8) control
registers. When the CPU is reset, IE is set to 0 (interrupts disabled).
Interrupt Priority (IPRI)
IPRI contains the current running interrupt priority. When an exception is
processed, the IPRI value is set to the exception number. See “Exceptions”
on page 16 for more information. For external hardware interrupts, the
IPRI value is set directly from the 6-bit hardware interrupt number. For
TRAP instructions, the IPRI field is set directly from the IMM6 field of
the instruction. For internal exceptions, the IPRI field is set from the
pre-defined 6-bit exception number.
A hardware interrupt is not processed if its internal number is greater
than or equal to IPRI or IE=0. A TRAP instruction is processed
unconditionally. When the CPU is reset, IPRI is set to 63 (lowest-priority).
IPRI disables interrupts above a certain number. For example, if IPRI is 3,
then interrupts 0, 1 and 2 will be processed, but all others (interrupts 3-63)
are disabled.
Current Window Pointer (CWP)
CWP points to the base of the sliding register window in the general-
purpose register file. Incrementing CWP moves the register window up 16
registers. Decrementing CWP moves the register window down 16
registers. CWP is decremented by SAVE instructions and incremented by
RESTORE instructions.
Only specialized system software such as register window-management
facilities should directly write values to CWP through WRCTL. Software
will normally modify CWP by using SAVE and RESTORE instructions.
When the CPU is reset, CWP is set to the largest valid value, HI_LIMIT.
This means in a 256 register file size, there will be 16 register windows.
After reset, the WVALID register (%ct12) is set to 0x01C1, i.e., LO_LIMIT
= 1 and HI_ LIMIT =14. See “WVALID (%ctl2)” on page 6 for more
information.