LSI 53C875A technical manual Chained Block Move Scripts Instruction

2.2.17.5 Chained Block Move SCRIPTS Instruction

A chained Block Move SCRIPTS instruction is primarily used to transfer consecutive data send or data receive blocks. Using the chained Block Move instruction facilitates partial receive transfers and allows correct partial send behavior without additional opcode overhead. Behavior of the chained Block Move instruction varies slightly for sending and receiving data.

For receive data (Data-In for initiator or Data-Out for target), a chained Block Move instruction indicates that if a partial transfer occurred at the end of the instruction, the WSR flag is set. The high-order byte of the last SCSI transfer is stored in the SCSI Wide Residue (SWIDE) register rather than transferred to memory. The contents of the SWIDE register should be the first byte transferred to memory at the start of the chained Block Move data stream. Since the byte count always represents data transfers to/from memory (as opposed to the SCSI bus), the byte transferred out of the SCSI Wide Residue (SWIDE) register is one of the bytes in the byte count. If the WSR bit is cleared when a receive data chained Block Move instruction is executed, the data transfer occurs similar to that of the regular Block Move instruction. Whether the WSR bit is set or cleared, when a normal block move instruction is executed, the contents of the SCSI Wide Residue (SWIDE) register are ignored and the transfer takes place normally. For “N” consecutive wide data receive Block Move instructions, the 2nd through the Nth Block Move instructions should be chained block moves.

For send data (Data-Out for initiator or Data-In for target), a chained Block Move instruction indicates that if a partial transfer terminates the chained block move instruction, the last low-order byte (the partial memory transfer) should be stored in the lower byte of the SCSI Output Data Latch (SODL) register and not sent across the SCSI bus. Without the chained Block Move instruction, the last low-order byte would be sent across the SCSI bus. The starting byte count represents data bytes transferred from memory but not to the SCSI bus when a partial transfer exists. For example, if the instruction is an Initiator chained Block Move Data Out of five bytes (and WSS is not previously set), five bytes are transferred out of memory to the SCSI controller, four bytes are transferred from the SCSI controller across the SCSI bus, and one byte is temporarily stored in the lower byte of the SCSI Output Data Latch (SODL) register waiting to be married with the first byte of the next Block Move instruction. Regardless of whether a chained Block Move or normal Block Move instruction is used, if the WSS bit is set at the start of a data

SCSI Functional Description 2-47