Result of

 

Bit 19

Compare

Action

 

 

 

0

False

Jump Taken

0

True

No Jump

1

False

No Jump

1

True

Jump Taken

 

 

 

CD

Compare Data

18

 

When this bit is set, the first byte received from the SCSI

 

data bus (contained in the SCSI First Byte Received

 

 

(SFBR) register) is compared with the Data to be

 

 

Compared Field in the Transfer Control instruction. The

 

Wait for Valid Phase bit controls when this compare

 

 

occurs. The Jump if True/False bit determines the

 

 

condition (true or false) to branch on.

 

CP

Compare Phase

17

 

When the LSI53C875A is in Initiator mode, this bit

 

 

controls phase compare operations. When this bit is set,

 

the SCSI phase signals (latched by SREQ/) are

 

 

compared to the Phase Field in the Transfer Control

 

 

instruction. If they match, the comparison is true. The

 

 

Wait for Valid Phase bit controls when the compare

 

 

occurs. When the LSI53C875A is operating in Target

 

 

mode and this bit is set it tests for an active SCSI SATN/

 

signal.

 

WVP

Wait for Valid Phase

16

 

If the Wait for Valid Phase bit is set, the LSI53C875A

 

 

waits for a previously unserviced phase before comparing

 

the SCSI phase and data.

 

 

If the Wait for Valid Phase bit is cleared, the LSI53C875A

 

compares the SCSI phase and data immediately.

 

DCMData Compare Mask[15:8]

The Data Compare Mask allows a SCRIPT to test certain bits within a data byte. During the data compare, if any mask bits are set, the corresponding bit in the SCSI First Byte Received (SFBR) data byte is ignored. For instance, a mask of 0b01111111 and data compare value of 0b1XXXXXXX allows the SCRIPTS processor to determine whether or not the high order bit is set while ignoring the remaining bits.

Transfer Control Instructions

5-31

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LSI 53C875A technical manual Compare Data, Compare Phase, Wvp, Wait for Valid Phase, DCMData Compare Mask158