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21264/EV68A Hardware Reference Manual
Cache and External Interfaces 4–31
System Port
If both the sender and the receiver are sampling at the same rate, these three principles
are sufficient to safely make point-to-point transfers using clock forwarding. However,
it is often desirable for systems to align clock-forwarded transactions on a slower
SYSCLK that is the basis of all non-processor system transactions.
The 21264/EV68A supports three ratios for SYSCLK to INT_FWD_CLK:
one-to-one (1-1), two-to-one (2-1), and four-to-one (4-1). Using one of these ratios, the
21264/EV68A starts transactions on SYSCLK boundaries. This ratio is programmed
into the 21264/EV68A using the Cbox CSR SYS_FRAME_LD_VECTOR[4:0]. This
ratio is independent of the frequency of FrameClk_H.
For data movement, the 21264/EV68A reacts to SysDc commands when they are
resolved into the 21264/EV68A’s clock domain. This occurs when the 21264/EV68A’s
INT_FWD_CLK unloads the SysDc command from the clock forwarding queue. This
moment is determined by the amount of delay programmed into the clock forwarding
silo (by way of Cbox CSR SYS_RCV_MUX_CNT_PRESET[1:0]). Thus, all the tim-
ing relationships are relative to this unload point in time, which will be referred to as
the point the command is perceived by 21264/EV68A.
4.7.8.2 Fast Data Mode
The 21264/EV68A is the default driver of the bidirectional SysData bus
1
.Asthe
21264/EV68A is processing WrVictim, ProbeResponse (only the hit case), and IOWB
commands to the system, accompanying data is made available at the clock-forwarded
bus.
Because there is a bandwidth difference between address (4 cycles) and data (8 cycles)
transfers, the 21264/EV68A tries to fully use fast data mode by delaying the next
SysAddOut write command until a fast data mode slot is available on the SysDataOut
bus.
SysDc commands (cache fill or explicit write commands) that collide with the fast data
on the SysData bus have higher priority, and so may interrupt the successful completion
of the fast transfer. Systems are responsible for detecting and replaying all interrupted
fast transfers. There are no gaps in a fast transfer and no data wrapping (the first cycle
contains QW0, addressed by PA[5:3] = 000).
The system must release victim buffers, and probe buffers and IOWB entries by send-
ing a SysDc command with the appropriate RVB/RPB bit for both successful fast data
transfers and for transfers that have been replayed. Fast data transfers have two parts:
1. SysAddOut command with the probe response, WrVictim, or Wr(I/O)
2. Data
The command precedes data by at least one SYSCLK period. Table 4–25 shows the
number of SYSCLK cycles between SysAddOut and SysData for all system clock
ratios (clock forwarded bit times) and system framing clock multiples.
1 The SysData bus contains SysData_L[63:0] and SysCheck_L[7:0].