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6 Altera Corporation
Overview
Condition Code Flags
Some instructions modify the condition code flags. These flags are the
four least significant bits of the status register as shown in Table 7.
ISTATUS (%ctl1)
ISTATUS is the saved copy of the STATUS register. When an exception is
processed, the value of the STATUS register is copied into the ISTATUS
register. This action allows the pre-exception value of the STATUS
register to be restored before control returns to the interrupted program.
See “Exceptions” on page 16 for more information. A return-from-trap
(TRET) instruction automatically copies the ISTATUS register into the
STATUS register. Interrupts are disabled (IE=0) when an exception is
processed. Before re-enabling interrupts, an exception handler must
preserve the value of the ISTATUS register. When the CPU is reset,
ISTATUS is set to 0.
WVALID (%ctl2)
WVALID contains two values, HI_LIMIT and LOW_LIMIT. When a
SAVE instruction decrements CWP from LOW_LIMIT to LOW_LIMIT –1
a register window underflow (exception #1) is generated. When a
RESTORE instruction increments CWP from HI_LIMIT to HI_LIMIT +1, a
register window overflow (exception #2) is generated. WVALID is
configurable and may be read-only or read/write. When the CPU is reset,
LO_LIMIT is set to 1 and HI_LIMIT is set to the highest valid window
pointer ((register file size / 16) – 2).
Table 7. Condition Code Flags
N Sign of result, or most significant bit
V Arithmetic overflow—set if bit 31 of 32-bit result is different from
sign of result computed with unlimited precision.
ZResult is 0
C Carry-out of addition, borrow-out of subtraction
1514131211109876543210
1514131211109876543210
HI_LIMIT LO_LIMIT