Mailbox Out Definition - Adaptec AHA-1740A Technical Reference Manual

EISA-to-Fast SCSI Host Adapter
Base
Addr
+0
+4
+8
+12
+16
+20
+24
+28
In this example, there are four MBOs and four MBIs. The Mailbox Count in the Mail-
box Initialization command was set to four. The base address is the address specified
by the Mailbox Address field.
The Adaptec BIOS initializes the AT DMA controller to accommodate Bus Master
DMA. If the host adapter BIOS is removed or disabled, the host DMA controller
must be initialized, via software, for Bus Master DMA operation. See the section
titled Description of Operation.

Mailbox Out Definition

The first byte of each MBO contains the mailbox status byte. The remaining three
bytes contain an address pointer to the first byte of a Command Control Block (CCB).
The CCB provides additional task control information. An MBO is free if the first
byte is zero. The host can make an entry in any free MBO and indicate that it is
filled out or completed by placing the proper MBO command in the first byte of the
MBO. After the MBO has been examined by the host adapter and all relevant infor-
mation obtained by the host adapter, the host adapter sets the MBO command byte
back to zero to allow the host to fill it again. For a multitasking operating system, it
is desirable that the number of mailboxes be sufficient to allow at least one mailbox
for each active independent task or activity.
CMD
CMD
00
CMD
00
Status
00
00
Note
Standard Mode Firmware Description
CCB 4 Pointer
CCB 2 Pointer
Free Entry
CCB 3 Pointer
Free Entry
CCB 1 Pointer
Free Entry
Free Entry
MBO 0
MBO 1
MBO 2
MBO 3
MBO 0
MBO 1
MBO 2
MBO 3
5-15