Specifications

10 Issue 5, August 1994 Acorn Enhanced Expansion Card
Acorn Enhanced Expansion Card
Figure 3: Extended ECld
Extended expansion card identity with paged
ROM
When the expansion card includes driver code in ROM,
there are several possibilities for implementing the ECId.
One example showing an EPROM with a paging register
is shown in Figure 5: Extended ECId with paged ROM on
page 11. Simplifications can be made where there is
only one page, or where a larger EPROM allows the
inclusion of the low byte of the ECId. (FIQ and/or IRQ
can be factored into the address space as in the
previous example).
IOC expansion cards
IOC expansion cards are controlled by the IOC. These
expansion cards may be accessed by one of four types
of cycle, designated slow, medium, fast, and
BD[0:7]
PS
LA13
OE
LA2
LA3
LA4
FIQ
IRQ
PRE
CS
D7
D0
A0
A4
32 X 8
PROM
synchronous. The cycles are mapped at different
addresses.
Once the cycle has started, MEMC may de-assert IORQ
(and hence IOC will de-assert RBE) in order to carry out
memory refresh or DMA operations. This is indicated by
the shaded area in Figure 6: IOC driving an expansion
card read cycle on page 11. If, when the IOC has
finished the cycle, the MEMC has not reasserted
IORQ
then the I/O cycle will be stretched until the MEMC is
ready to complete the cycle. This does not, however,
alter the cycle the expansion card sees, because the
cycle is in effect finished before the stretching takes
place. In the case of a write the WBE and PS have
already been de-asserted. In the case of a read the data
from the expansion card has already been latched into
the data buffers by BL.
Figure 6 shows how the IOC generates a fast expansion
card read: the IOC generates the expansion card read,
write and select strobes, and also controls the IOGT and
BL signals. The expansion card read, write and select
signals are timed with respect to the signal CLK8 and no
relationship between REF8M and CLK8 can be
assumed.
Figure 7: Stretched expansion card read cycle on
page 12 shows the same cycle, but this time the MEMC
has de-asserted IORQ at the time when the IOC is about
to finish the cycle. Accordingly, the cycle is stretched by
one REF8M (the shaded region), but note that the
access to the expansion card has not been stretched.
Figure 4: Chunk directory structure
Extended PI
Code Chunk 2
PAGE 0 PAGE 1
8
16
24
Int. Status Pointers
0
four zero bytes
0
Loader
operating system #0
Loader
operating system #1
Chunk Dir entry 2
Chunk Dir entry 1
Chunk Dir entry 0
Bytes
32
40
44
Chunk Dir entry 3
Chunk Dir entry 4
four zero bytes
Code Chunk 3
Code Chunk 3
(continued)
Code Chunk 4
Podule