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Instruction Set of the I/O Processor
I/O Instructions
SYM53C825A/825AE Data Manual
6-11
Bit 25
Table Indirect Mode
When this bit is set, the 24-bit signed value in
the DBC register is added to the value in the
DSA register, used as an offset relative to the
value in the Data Structure Base Address
(DSA) register. T he SCNT L3 value, SCSI ID,
synchronous offset and synchronous period are
loaded from this address. Prior to the start of
an I/O, the DSA should be loaded with the
base address of the I/O data structure. T he
address may be any address on a longword
boundary. After a Table Indirect op code is
fetched, the DSA is added to the 24-bit signed
offset value from the op code to generate the
address of the required data; both positive and
negative offsets are allowed. A subsequent
fetch from that address brings the data values
into the chip.
SCRIPT S can directly execute operating sys-
tem I/O data structures, saving time at the
beginning of an I/O operation. T he I/O data
structure can begin on any longword boundary
and may cross system segment boundaries.
T here are two restrictions on the placement of
data in system memory:
1. T he I/O data structure must lie within the 8
MB above or below the base address.
2. An I/O command structure must have all four
bytes contiguous in system memory, as shown
below. T he offset/period bits are ordered as in
the SX FER register. T he configuration bits are
ordered as in the SCNT L3 register.
T his bit should only be used in conjunction
with the Select, Reselect, Wait Select, and Wait
Reselect instructions. Bits 25 and 26 may be
set individually or in
combination:
Direct
Uses the device ID and physical address in the
instruction.
Table Indirect
Uses the physical jump address, but fetches data
using the table indirect method.
Relative
Uses the device ID in the instruction, but treats
the alternate address as a relative jump
Config
ID
Offset/
period
(00)
Bit 25
Bit 26
Direct
Table Indirect
Relative
Table Relative
0
0
1
1
0
1
0
1
Com-
mand
ID
Not Used
Not Used
Absolute Alternate Address
Command
Table Offset
Absolute Alternate Address
Com-
mand
ID
Not Used
Not
Used
Alternate Jump Offset