
Chapter 6. Additional Bus Configurations
6-1
Chapter 6
Additional Bus Configurations
60
60
Chapters 2 through 5 describe basic 60x bus operations. However some processors support
additional bus functionality, including the following:
No-data retry mode (referred to as no-DRTRY mode).—This mode allows DRTRY
to be disabled in the 603 and 604e, which in turn allows data to be forwarded one
bus cycle sooner than if DRTRY is enabled. (No-DRTRY mode is implemented on
the 604e, but not on the 604; see data streaming mode below.)
Data streaming mode—Data streaming is the ability to begin data tenure after a
previous data tenure with no dead cycles between. Data streaming is implemented
on 604s. (Note that in 604 documentation, this was called fast-L2/data streaming
mode and no-DRTRY/data streaming mode, although there is no relation to the no-
DRTRY mode described above.)
32-bit data bus mode—The 603 supports an optional 32-bit data bus mode, in which
the processor uses only byte lanes 0–3 for a data transfer, therefore allowing a
maximum of 32 bits of data to be transferred per bus clock.
Reduced pinout mode—The 603 provides an optional reduced-pinout mode that
disables DL[0–31], DP[0–7], AP[0–3], APE, DPE, and RSRV for reduced power
consumption. The 32-bit data bus mode is implicitly selected when reduced-pinout
mode is enabled.
Direct-store mode, which provides an alternative method for I/O bus operations, is
described in Chapter 7, “Direct-Store Interface.”
6.1 No-DRTRY Mode (603 and 604e)
The 603 family and 604e processors provides a way to disable the use of the data retry
function. No-DRTRY mode allows data to be forwarded during load operations to the
internal processor one bus cycle sooner than with normal bus protocol.
The 60x bus protocol specifies that, during load operations, the memory system normally
can cancel data that the master read on the bus cycle after TA was asserted. On 603 and 604e
processors, this late cancellation requires any data loaded at the bus interface to be held one
additional bus clock to verify that the it is valid before forwarding it to the internal CPU.
For systems that do not use the DRTRY function, no-DRTRY mode eliminates this one-
cycle stall and allows data to pass to the internal CPU immediately when TA is recognized.