Compaq 21264 - Manual (Page 104)

4–16 Cache and External Interfaces Alpha 21264/EV67 Hardware Reference Manual
System Port
If the ChangeToDirty command succeeds, the STx_C enters the writable state, and the
Mbox locks the Dcache line. The Mbox does not release the Dcache line until the
STx_C data is transferred to the Dcache. This ensures that no other agent, by way of a
probe, can take the block before the STx_C can update the locked block.
4.6.4 Managing Speculative Store Issues with Multiprocessor Systems
The 21264/EV67 provides two mechanisms to manage an inherent potential side effect
of speculative execution with multiprocessor systems — a livelock condition caused by
a speculative store that misses in one processor affecting the execution of a LDx_L/
STx_C pair in another processor. The potential livelock condition in multiprocessor
systems can be effectively controlled by placing processors in a conservative mode,
where speculative store MAFs are blocked. The 21264/EV67 manages conservative
mode with the Mbox IPR, M_CTL[SMC], described in Table 5–19.
• M_CTL[SMC] can be set to place the 21264/EV67 in full-time conservative mode.
• M_CTL[SMC] can be set to place the 21264/EV67 in periodic conservative mode,
timed by two counters: an 8-bit primary counter that tracks branch mispredicts and
conditional branch retires, and a backup counter that places the 21264/EV67 in
conservative mode for a period of 16K cycles every 2 million cycles.
The 8-bit counter is enabled by placing M_CTL[SMC] in periodic conservative
mode. The backup counter takes effect whenever the 8-bit counter is enabled. Fur-
ther, the backup counter can be reset to 0 by clearing a previously set
M_CTL[SMC], allowing synchronization between processors.
4.7 System Port
The system port is the 21264/EV67’s connection to either a memory or I/O controller or
to a shared multiprocessor system controller. System port interface signals are shown in
Figure 4–4.
The system port supports transactions between the 21264/EV67 and the system. Sys-
tems must receive and drive signals that are asserted low. Transaction commands are
communicated on signal lines SysAddOut_L[14:0] (21264/EV67-to-system) and
SysAddIn_L[14:0] (system-to-21264/EV67). Transaction data is transferred on a bidi-
rectional data bus over pins SysData_L[63:0] with ECC on pins SysCheck_L[7:0].