Table 5.2 SCSI Information Transfer Phase

MSG

C_D

I_O

SCSI Phase

 

 

 

 

0

0

0

Data-Out

0

0

1

Data-In

0

1

0

Command

0

1

1

Status

1

0

0

Reserved

1

0

1

Reserved

1

1

0

Message-Out

1

1

1

Message-In

 

 

 

 

TC[23:0] Transfer Counter[23:0]

This 24-bit field specifies the number of data bytes to be moved between the LSI53C875A and system memory. The field is stored in the DMA Byte Counter (DBC) register. When the LSI53C875A transfers data to/from memory, the DBC register is decremented by the number of bytes transferred. In addition, the DMA Next Address (DNAD) register is incremented by the number of bytes transferred. This process is repeated until the DBC register is decremented to zero. At this time, the LSI53C875A fetches the next instruction.

If bit 28 is set, indicating table indirect addressing, this field is not used. The byte count is instead fetched from a table pointed to by the Data Structure Address (DSA) register.

5-12

SCSI SCRIPTS Instruction Set

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LSI 53C875A technical manual Scsi Information Transfer Phase, TC230 Transfer Counter230, Scsi Phase