Specifications

Appendix B Issue 5, August 1994 B-1
DMA Extended Bus Interface
Appendix B: DMA Extended Bus Interface
Introduction
This appendix describes the DMA Extended Bus
Interface (DEBI). DEBI is presently fully supported on the
first two expansion slots of the Risc PC 600, although
future machines may support all or only part of the
interface. The DEBI bus includes two types of interface,
DMA and the new Extended Address Space Interface
(EASI). Any expansion card which includes a DMA
interface should be termed a ‘DEBI’ card, and any
expansion card which supports EASI but not DMA should
be termed an ‘EASI’ card.
DMA
The Direct Memory Access (DMA) transfer can be byte,
half-word or word wide and appear on the expansion card
as one of four different transfer cycle types. The four
cycle types/speeds which can be used are types A, B, C
and D. The slowest of these is A and the quickest is D. As
mentioned in the section entitled DEBI and EASI
expansion cards on page 3 of the Acorn Enhanced
Expansion Card Specification, the data buffer size of the
DMA channel means that data can be transferred in
continuous blocks of between 1 byte and 4KB. Each DMA
channel has two such data buffers, both up to 4KB in
size, so data transfer speeds can be optimised by
performing DMA transfers from one buffer whilst
programming the other buffer ready for the next block
transfer. The use of two buffers for large DMA transfers
will achieve a close to continuous flow of data in 4KB
page sizes.
Timing information for all DMA cycle types is shown on
page B-4 of this appendix. A full description of how the
DMA channel is programmed is beyond the scope of this
document and can be found in the RISC OS 3
Programmer’s Reference Manual, Vol. 5 (version 3.5
supplement) part number 0497,551.
As suggested above, expansion cards containing any
DMA capabilities should be termed ‘DEBI’ expansion
cards. Expansion cards containing both DMA and EASI
capabilities should also be termed ‘DEBI’ expansion
cards.
DEBI
EASI (Extended Address
Space Interface)
DMA
EASI
EASI accesses can be byte, half-word and word wide
transfers and may be of two different access cycle
speeds. The two cycle types/speeds are type A and type
C, type C being the shorter, and therefore quicker, cycle
of the two.
Each expression slot supports up to 16MB of extended
address space which can be addressed as byte or word
wide, hence the use of half word data transfers to EASI
space will only utilize 50% of the area.
The two access cycle types can be stretched by the
expansion card to match slower peripherals. To stretch
an EASI transfer cycle the expansion card must assert
the Ready line soon after the transfer cycle begins and
keep the line asserted until the expansion card is ready
to complete the cycle – see Extended address space
(EASI) timings on page B-3 for detailed timing.
As already mentioned, expansion cards containing an
EASI type interface with no DMA capabilities should be
termed ‘EASI’ expansion cards.
Major features of DEBI
The major features of the DEBI are as follows:
• 32 bit data bus allowing byte, half-word and word wide
transfers
• An additional 16MB of address space per expansion
slot
• DMA is available on selected slots
• A wide range of access cycle times
• 6MB/second transfer rate
• Intel-compatible control signals
It is anticipated that all DEBI and EASI expansion cards
will be of the single Eurocard form factor. This enables
card guides to be provided in the computer so making it
easier for the user to install cards. Single Eurocard width
also improves the EMC qualities of the computer and
improves the integrity of the fire enclosure.