
Chapter 3. Memory Access Protocol
3-3
processors support one- and four-beat transfers. Figure 3-2 shows a data transfer that
consists of a single-beat transfer of as many as 64 bits. Four-beat burst transfers of 32-byte
cache blocks are also supported, and on the 603, eight-beat bursts can be used to transfer
an eight-word cache block when the processor is operating in 32-bit data bus mode. Burst
operations require data transfer termination signals for each beat.
Address-only transactions are used to broadcast synchronizing and cache control
operations, especially in multiprocessor systems.
Figure 3-2. Overlapping Tenures on the Processor Bus for a Single-Beat Transfer
Basic functions of the address and data tenures are as follows:
Address tenure
— Arbitration: During arbitration, address bus arbitration signals are used to gain
mastership of the address bus.
— Transfer: After mastership is obtained, the address bus master transfers the
address, transfer attributes, and parity information on the address bus. Address
signals and transfer attribute signals control the address transfer. Address parity
and address parity error signals ensure the integrity of the address transfer.
— Termination: After the address transfer, the system signals that the address tenure
is complete or that it must be repeated.
Data tenure
— Arbitration: To begin a data tenure, the master arbitrates for data bus mastership.
— Transfer: After mastership is obtained, the data bus master samples the data bus
for read operations or drives the data bus for write operations. The data parity and
data parity error signals ensure the integrity of the data transfer.
— Termination: Data termination signals are required after each data beat in a data
transfer. In single-beat transactions, data termination signals also indicate the
end of the tenure, while in burst accesses, data termination signals apply to
individual beats and indicate the end of the tenure only after the final data beat.
ARBITRATION
TRANSFER
TERMINATION
ADDRESS TENURE
ARBITRATION
SINGLE-BEAT TRANSFER
TERMINATION
DATA TENURE
INDEPENDENT ADDRESS AND DATA