Directory Base Register - Intel i86W Manual

REGISTERS
• OF (Overflow Flag) is set by
adds, addu, subs,
and
subu
when integer overflow
occurs. For
adds
and
subs,
OF is set if the carry from bit 31 is different than the carry
from bit 30. For
addu,
OF is set if there is a carry from bit 31. For
subu,
OF is set if
there is no carry from bit 31. Under all other conditions, it is cleared by these instruc-
tions. OF controls the function of the
intovr
instruction (refer to Chapter 5).
3.5 DATA BREAKPOINT REGISTER
The data breakpoint register
(db)
is used to generate a trap when the i860 microproces-
sor accesses an operand at the address stored in this register. The trap is enabled by BR
and BW in
psr.
When comparing, a number of low order bits of the address are ignored,
depending on the size of the operand. For example, a 16-bit access ignores the low-order
bit of the address when comparing to
db;
a 32-bit access ignores the low-order two bits.
This ensures that any access that overlaps the address contained in the register will
generate a trap. The trap occurs before the register or memory update by the load or
store instruction.
3.6 DIRECTORY BASE REGISTER
The directory base register
dirbase
(shown in Figure 3-4) controls address translation,
caching, and bus options.
• ATE (Address Translation Enable), when set, enables the virtual-address translation
algorithm described in Chapter 4. The data cache must be flushed before changing
the ATE bit.
• DPS (DRAM Page Size) controls how many bits to ignore when comparing the cur-
rent bus-cycle address with the previous bus-cycle address to generate the NENE#
signal. This feature allows for higher speeds when using static column or page-mode
ADDRESS TRANSLATION ENABLE - - - - - - - - - - - - - - - ,
DRAM PAGE SIZE
BUS LOCK
-----------------------------~
I-CACHE, TLB INVALIDATE
-------------.j
(RESERVED)
1
CODE SIZE 8-BIT - - - - - - - - - - - - - - - - - - ,
REPLACEMENT BLOCK
!
REPLACEMENT CONTROL
~
J
12
DIRECTORY TABLE BASE (DTB) RC
Figure 3-4. Directory Base Register
3-6
240329i