AMD AMD5K86 - TABLE 5-5. Relation of BE7-BE0 to Other Signals

AMD~
AMD51J6 Processor Technical Reference Manual
18524BjO-Mar1996
5.2.8
Summary
8E7-BEO (Byte Enables)
Output
The eight bits of BE7-BEU, when cleared to 0, validate the
eight bytes driven on D63-DO. In this way, BE7-BEU expands
on the function of address bits A2-AO, which do not exist on
the A31-A3 address bus. BE7-BEU also help differentiate the
special bus cycles.
Driven and Floated
The processor drives BE7-BEU from the clock in which ADS is
asserted until the last expected BRDY of the bus cycle. The
processor floats BE7-BEU one clock after system logic asserts
BUFF and in the same clock that the processor asserts HLDA.
Details
5-J4
BE7-BEU is driven with the address and cycle definition out-
puts (DiC, M/IO and WIR) during memory cycles (including
cache writethroughs and writebacks), I/O cycles, locked cycles,
special bus cycles, and interrupt acknowledge operations in
the normal operating modes (Real, Protected, and Virtual-
8086) and in SMM, or while PRDY is asserted. While AHOLD is
asserted, BE7-BEU is driven only to complete a bus cycle that
had been initiated before AHOLD was asserted, or for inquire
cycle writebacks. During the Shutdown, Halt, and Stop Grant
states, BE7-BEU is driven only for inquire cycle writebacks.
BE7-BEU is not driven during the Stop Clock state, or while
BUFF, HLDA, RESET, or INIT is asserted.
Table 5-5 shows the relationship between BE7-BEU, D63-DO,
DP7-DPO,
and the effective relationship with A2-AO, the non-
existent low address bits. The BE7-BEU signals expand on the
function of A2-AO; BE7-BEU allow the processor to address
any or all eight bytes indicated by A31-A3, whereas A2-AO, if
they existed, would only address one of eight bytes.
During single-transfer memory cycles and all I/O cycles, the
processor drives BE7-BEU to identify all of the bytes desired
for the transfer. System logic must return valid data in those
byte lanes of D63-DO.
During burst reads (CACHE and KEN both asserted with the
first BRDY of a memory read), the processor drives BE7-BEO
with ADS to identify the bytes of the desired instruction or
operand. The processor drives BE7-BEO with the desired bytes
at that time because it does not yet know whether the read will
be a single-transfer or a burst-this depends on how system
Bus Interface