VR4102 datasheet(185/701 Pages) NEC

185 / 701 page

CHAPTER 6 EXCEPTION PROCESSING

185

6.4.10 Bus Error Exception

Cause

A Bus Error exception is raised by board-level circuitry for events such as bus time-out, local bus parity errors,

and invalid physical memory addresses or access types. This exception is not maskable.

A Bus Error exception occurs only when a cache miss refill, uncached reference, or unbuffered write occurs

synchronously. In other words, it occurs when an illegal access is detected during BCU read.

For details of illegal accesses, refer to 10.4.6 Illegal Access Notification.

Processing

The common interrupt vector is used for a Bus Error exception. The IBE or DBE code in the ExcCode field of the

Cause register is set, signifying whether the instruction caused the exception by an instruction reference, load

operation, or store operation.

The EPC register contains the address of the instruction that caused the exception, unless it is in a branch delay

slot, in which case the EPC register contains the address of the preceding branch instruction and the BD bit of the

Cause register is set to 1.

Servicing

The physical address at which the fault occurred can be computed from information available in the System

Control Coprocessor (CP0) registers.

If the IBE code in the Cause register is set (indicating an instruction fetch), the virtual address is contained

in the EPC register (or 4 + the contents of the EPC register if the BD bit of the Cause register is set to 1).

If the DBE code is set (indicating a load or store), the virtual address of the instruction that caused the

exception (the address of the preceding branch instruction if the BD bit of the Cause register is set to 1) is

saved to the EPC register (or 4 + the contents of the EPC register if the BD bit of the Cause register is set to

1).

The virtual address of the load and store instruction can then be obtained by interpreting the instruction. The

physical address can be obtained by using the TLBP instruction and reading the EntryLo register to compute the

physical page number.

At the time of this exception, the kernel reports the UNIX SIGBUS (bus error) signal to the current process, but

the exception is usually fatal.

VR4102 Datasheet(PDF) 185 Page - NEC