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21020285 D User’s Guide
Chapter 2 Hardware Essentials
Page 2 - 8 Address Buffer Port
Address Buffer
Port
The Address Buffer Port is a write-only port. It is used when you wish to execute
a command using a Single Command Structure (SCS). It is loaded with: the
starting system address of the structure; an
address modifier value to be used when reading the Parameter Block contained
in the structure; and a byte of control information through which you may
instruct the RF3880 adapter about subsequent operation.
The Address Buffer Port register is three words deep and 16 bits wide. To pass
all three words requires three writes to the port:
• The first write must contain the Control byte and Address Modifier.
• The second write contains the Most Significant Word (MSW) of the
Single Command Structure address in system memory.
• The last write contains the Least Significant Word (LSW) of the Single
Command Structure address.
The order of these writes is illustrated in the following figure:
A description of each of the fields of the Address Buffer Port follows.
Address Modifier Field
The Address Modifier (AM) Field of the Address Buffer Port is used to specify
the Address Modifier value that the RF3880 adapter should use when reading
the first Parameter Block.
Bits 15 8 7 0
1st Write Control AM Bits for SCS
2nd Write Single Command Structure Address: MSW
3rd Write Single Command Structure Address: LSW
Table 6: Address Buffer Port Writes