am53c94 Advanced Micro Devices, am53c94 Datasheet - Page 18

• Manufacturer Part Number: am53c94
• Description: High Performance Scsi Controller
• Manufacturer: Advanced Micro Devices

CMDREG – Bit 7 – DMA – Direct Memory Access
The DMA bit when set notifies the device that the com-
mand is a DMA instruction, when reset it is a non-DMA
instruction. For DMA instructions the Current Transfer
Count Register (CTCREG) will be loaded with the con-
tents of the Start Transfer Count Register (STCREG).
The data is then transferred and the CTCREG is decre-
mented for each byte until it reaches zero.
18
Initiator Commands
Target Commands
Idle Commands
General Commands
CMD6
CMD6
CMD6
CMD6
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
0
0
0
0
AMD
CMD5
CMD5
CMD5
CMD5
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
CMD4
CMD4
CMD4
CMD4
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CMD3
CMD3
CMD3
CMD3
0
0
0
1
1
1
0
0
0
0
0
0
0
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
P R E L I M I N A R Y
CMD2
CMD2
CMD2
CMD2
Am53C94/Am53C96
0
0
0
0
0
0
0
0
0
0
1
1
1
0
0
0
0
1
0
0
0
0
1
1
1
0
0
0
0
CMD1
CMD1
CMD1
CMD1
CMDREG – Bits 6:0 – CMD 6:0 – Command 6:0
These command bits decode the commands that the
device needs to perform. There are a total of 29
commands grouped into four categories. The groups
are Initiator Commands, Target Commands, Selection/
Reselection Commands and General Purpose Com-
mands.
0
1
0
1
1
0
0
1
1
0
0
1
0
0
1
1
0
0
1
1
0
0
1
0
1
1
0
0
0
CMD0
CMD0
CMD0
CMD0
0
1
0
0
0
1
0
1
0
1
0
1
1
0
1
0
1
0
0
1
0
1
0
1
0
0
1
0
1
Command
Information Transfer
Initiator Command Complete Steps
Message Accepted
Transfer Pad Bytes
Set ATN
Reset ATN
Command
Send Message
Send Status
Send Data
Disconnect Steps
Terminate Steps
Target Command Complete Steps
Disconnect
Receive Message Steps
Receive Command
Receive Data
Receive Command Steps
DMA Stop
Command
Reselect Steps
Select without ATN Steps
Select with ATN Steps
Select with ATN and Stop Steps
Enable Selection/Reselection
Disable Selection/Reselection
Select with ATN3 Steps
Command
No Operation
Clear FIFO
Reset Device
Reset SCSI Bus