CENTRAL PROCESSING UNIT (CPU)
7902 Group User’s Manual
2-12
2.2 Bus interface unit (BIU)
4
3
2
1
Table 2.2.3 Number of bytes to be fetched into instruction queue buffer
(2)
Instruction prefetch from external memory
With external data bus width = 16 bits (BYTE = Vss level)
8 bytes are fetched at a time from 8-byte boundaries. (See Figure 2.2.3-(b): 4 successive accesses).
At branch, regardless of the low-order 2 bits’ contents (A1, A0) of the branch destination address,
4 bytes are fetched at a time from 4-byte boundaries. (See Figure 2.2.3-(c): 2 successive accesses).
At this time, the number of prefetched bytes varies according to the branch destination address.
The operations succeedingly performed vary according to the address to be fetched next, as
follows:
When the address is at an 8-byte boundary, 8 bytes will be fetched from the next time. (See
Figure 2.2.3-(b): 4 successive accesses).
When the address is at a 4-byte boundary, 4 bytes will be fetched. (See Figure 2.2.3-(c): 2
successive accesses). Then, from the next time, 8 bytes will be fetched. (See Figure 2.2.3-
(b): 4 successive accesses).
With external data bus width = 8 bits (BYTE = Vcc level)
4 bytes are fetched at a time from 4-byte boundaries. (See Figure 2.2.3-(b): 4 successive accesses).
At branch, when the branch destination is at an even-numbered address, 2 bytes are fetched from
even-numbered addresses; when the branch destination is at an odd-numbered address, 2 bytes
are fetched from the address given by (odd-numbered address – 1). (See Figure 2.2.3 (c): 2
successive accesses);
The operations succeedingly performed vary according to the address to be fetched next, as
follows:
When the address is at a 4-byte boundary, 4 bytes will be fetched from the next time. (See
Figure 2.2.3-(b): 4 successive accesses).
When the address is at an even-numbered one, 2 bytes will be fetched. (See Figure 2.2.3-
(c) : 2 successive accesses). Then, from the next time, 4 bytes will be fetched. (See Figure
2.2.3-(b): 4 successive accesses).
AD0 (A0)
0
1
0
1
AD1 (A1)
0
1
AD1 (A1)
0
Low-order 2 bits of branch destination
address
Low-order 2 bits of address to be
output onto address bus
Number of bytes to be
fetched into instruction
queue buffer
AD0 (A0)
0