SAA7391H Philips Semiconductors, SAA7391H Datasheet - Page 57

• Manufacturer Part Number: SAA7391H
• Description: ATAPI CD-R block encoder/decoder
• Manufacturer: Philips Semiconductors

Philips Semiconductors
7.7.2
Memory is mapped as a 12-bit block number and a 12-bit
offset into that block. The block oriented memory structure
permits the use of 16-bit pointers in software, minimising
the overhead of accessing memory. The address can be
found from the following equation:
The sub-CPU sees the SAA7391 as a memory mapped
peripheral, with control and status registers appearing in
the highest 256 bytes of the external address space
(PDATA space).
The phrase (PDATA space) is meant to imply that the code
will access registers most efficiently if the PDATA
(8051 port P2) pointer is set to point at the register space
of the SAA7391. If the PDATA space is better used as
context switching space then it can be used for that
purpose.
All registers and RAM are accessible in the XDATA space
at all times, the PDATA is just a movable 256 byte window
with faster access into XDATA.
1997 Aug 01
address
handbook, full pagewidth
ATAPI CD-R block encoder/decoder
(1) Address decoded in upper 16 kbytes indicates access to RAM or SAA7391 registers, RAM access begins, SAA7391 accesses RAM, stops clock
(2) 8051 microcontroller continues.
only if SAA7391 asserts RD or WR before access complete.
B
=
UFFER MEMORY ORGANISATION
block_number 2560
XDA15
sub-CPU clock
sub-CPU ALE
XDA8
RD/WR
to
XDA0 to XDA7, XDA8 to XDA15 latched
+
offset
Fig.10 Fast RAM access mode timing.
(1)
57
The lowest 56 kbytes of the 8051 external address data
space is mappable as two windows into the memory of
52 kbytes and 4 kbytes, on any user-specified 256 byte
boundary within the RAM. This is usable as scratchpad
RAM.
The two pages permits the paging of process context
information for use with a multi-tasking kernel, while still
keeping some global variables.
The next 7.5 kbytes is mapped as a window into memory
starting at a user-specified block number. This is usable for
accessing block data, subcode information, error corrector
status and block headers.
The 64 kbytes memory mapping is shown in Fig.11.
XDD0 to XDD7
(2)
Objective specification
MGK517
SAA7391