when an interrupt occurs. These bits are unstable while data is being transferred between the two cores; once the chip has stopped transferring data, these bits are stable.

The DMA FIFO (DFIFO) register counts the number of bytes transferred between the DMA core and the SCSI core. The DMA Byte Counter (DBC) register counts the number of bytes transferred across the host bus. The difference between these two counters represents the number of bytes remaining in the DMA FIFO.

The following steps determine how many bytes are left in the DMA FIFO when an error occurs, regardless of the transfer direction:

1.Subtract the seven least significant bits of the DMA Byte Counter (DBC) register from the 7-bit value of the DMA FIFO (DFIFO) register.

2.AND the result with 0x7F for a byte count between zero and 64.

Note: To calculate the total number of bytes in both the DMA FIFO and SCSI logic, see Section 2.5.1.1, “Data Paths,” in Chapter 2, “Functional Description.”

Register: 0x21 (0xA1)

Chip Test Four (CTEST4)

Read/Write

7

6

5

4

3

2

 

0

BDIS

ZMOD

ZSD

SRTM

MPEE

 

FBL[2:0]

 

 

 

 

 

 

 

 

 

0

0

0

0

0

0

0

0

 

 

 

 

 

 

 

 

BDIS

Burst Disable

 

 

 

7

When set, this bit causes the LSI53C810A to perform back-to-back cycles for all transfers. When this bit is cleared, back-to-back transfers for opcode fetches and burst transfers for data moves are performed. The handling of opcode fetches is dependent on the setting of the Burst Opcode Fetch bit in the DMA Mode (DMODE) register.

5-34

Operating Registers

Page 108
Image 108
LSI 53C810A technical manual Register 0x21 0xA1, Chip Test Four CTEST4 Read/Write, Bdis, Burst Disable