A typical mailbox structure is:Base Adr+0 CMD CCB 4 Pointer MBO 0+4 CMD CCB 2 Pointer MBO 1+8 00 Free Entry MBO 2+12 CMD CCB 3 Pointer MBO 3+16 00 Free Entry MBI 0+20 Status CCB 1 Pointer MBI 1+24 00 Free Entry MBI 2+28 00 Free Entry MBI 3In this example there are four MBOs and four MBIs. The first byte of each MBO con-tains the MBO Command byte. The remaining three bytes point to a Command Con-trol Block (CCB). The CCB provides all the rest of the information needed tocomplete a task. An MBO is available to accept a new entry if the first byte is zero.The first byte of each MBI contains the status of a completed task. The remainingthree bytes point to the CCB of the completed task. An MBI is free if the Status byteis zero. Mailboxes may point to CCBs controlling initiator tasks, controlling targettasks, or controlling error recovery tasks.Command Control BlockA Command Control Block provides the information required to control a SCSI com-mand sequence. The block contains pointers to the data area to be used by the com-mand. It contains areas for presenting status of both the host adapter and theaddressed SCSI device. In addition, it contains the SCSI Command Descriptor Blockdefining the action to be taken by the addressed SCSI device. An error informationbuffer area is also provided.A Command Control Block is also used to service an operation requested by anotherinitiator when the AHA-1740A/1742A/1744 is being addressed as a SCSI Processordevice. The CCB is defined completely in Chapter Five, Standard Mode FirmwareDescription.adaptec AHA-1740A/1742A/17442-6
