Overlapped Instruction Fetch And Execution - Intel iAPX 86/88 User Manual

8086/8088 CPU
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GENERATION
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8086/8088
MICROPROCESSOR
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I II
III
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INSTRUCTION STREAM
1st INSTRUCTION (ALREADY FETCHED):
EXECUTE AND WRITE RESULT
2nd INSTRUCTION:
EXECUTE ONLY
3.d INSTRUCTION:
READ OPERAND AND EXECUTE
4th INSTRUCTION:
(UNDEFINED)
5th INSTRUCTION:
(UNDEFINED)
Figure 1-5 Overlapped Instruction Fetch and Execution
1. If AF (the auxiliary flag) is set, there has been a carry
out of the low nibble into the high nibble or a borrow
from the high from the high nibble into the low nibble
of an 8-bit quantity (low-order byte of a 16-bit
quantity). This flag is used by decimal arithmetic in-
structions.
2. If CF (the carry flag) is set, there has been a carry out
of, or a borrow into, the high-order bit of the result
(8-or 16- bit). The flag is used by instructions that add
and subtract multibyte numbers. Rotate instructions
can also isolate a bit in memory or a register by plac-
ing it in the carry flag.
3. If OF (the overflow flag) is set, an arithmetic overflow
has occurred; that is, a significant digit has been lost
because the size of the result exceeded the capacity of
1-6
its destination location. An Interrupt On Overflow in-
struction is available that will generate an interrupt in
this situation.
4. If SF (the sign flag) is set, the high-order bit of the
result is a 1. Since negative binary numbers are repre-
sented in the 8086 and 8088 in standard two's comple-
ment notation, SF indicates the sign of the result
(0
=
positive, 1
=
negative).
5.
If the PF (the parity flag) is set, the result has even
parity, an even number of I-bits. This flag can be used
to check for data transmission errors.
6. If ZF (the zero flag) is set, the result of the operation
is O.
Three additional control flags (see Figure 1-8) can be set
and cleared by programs to alter processor operations:
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