Compaq EV68A specifications System Port, 16Cache and External Interfaces

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/EV68A 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 multipro- cessor systems can be effectively controlled by placing processors in a conservative mode, where speculative store MAFs are blocked. The 21264/EV68A manages conser- vative mode with the Mbox IPR, M_CTL[SMC], described in Table 5–19.

•M_CTL[SMC] can be set to place the 21264/EV68A in full-time conservative mode.

•M_CTL[SMC] can be set to place the 21264/EV68A in periodic conservative mode, timed by two counters: an 8-bit primary counter that tracks branch mispre- dicts and conditional branch retires, and a backup counter that places the 21264/ EV68A 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.

4.7System Port

The system port is the 21264/EV68A’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/EV68A 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/EV68A-to-system) and SysAddIn_L[14:0] (system-to-21264/EV68A). Transaction data is transferred on a bidirectional data bus over pins SysData_L[63:0] with ECC on pins SysCheck_L[7:0].

4–16Cache and External Interfaces 21264/EV68A Hardware Reference Manual