Specifications

2-32 TLSB Bus
structure. Software may choose to reuse mailboxes (for example, multiple
reads from the same CSR), or it may maintain templates that are copied to
the mailbox.
Byte masks may be needed by some hardware devices for correct operation
of a CSR read as well as a CSR write. A bit is set in the mailbox MASK
field to disable the corresponding byte location to be read or written. See
the Alpha SRM for more details on the use of the mailbox.
2.3.3 Window Space I/O
CSR read and write commands directed at addresses in regions 8 through
C of the TLSB CSR address space provide direct access to remote CSRs in
devices attached to I/O subsystems through the I/O nodes on the TLSB.
This is an alternate method of accessing remote CSRs.
Each I/O node determines if it should respond to a window space CSR ac-
cess by comparing the CSR address space region to its physical node ID.
The I/O node acknowledges all addresses within the region, whether or not
the remote register exists. A CSR write command is a request to write the
remote CSR. A CSR read command is a request to read the remote CSR.
Mapping of the address to a remote CSR is implementation specific.
2.3.3.1 CSR Write Transactions to Remote I/O Window Space
A CSR write command to node 4 to 8 I/O window space causes the ad-
dressed I/O module to initiate a write to a remote CSR. The CPU can con-
sider the write operation complete as soon as the write data is transferred
on the TLSB to the I/O node.
As soon as the I/O node consumes the address and data and can free the
buffer, it issues a CSR write command to a Window Space DECR Queue
Counter (TLWSDQRn) register in local CSR broadcast space, where n is
the physical node number of the I/O node. The I/O node is then ready to
receive another window space request. If the I/O node acknowledges the
CSR write command, it must also cycle the data bus and provide data with
good ECC. The data should be considered Unpredictable as the value has
no significance. The I/O node may choose not to acknowledge the com-
mand and save data bus cycles.
If the I/O node detects an error while writing the remote CSR, it sets a
node-specific error bit and generates an IPL17 interrupt to the TLSB.
2.3.3.2 CSR Read Transactions to Remote I/O Window Space
A CSR read command to node 4 to 8 I/O window space causes the ad-
dressed I/O module to initiate a read of a remote CSR. The virtual ID of
the CPU initiating the read transaction must be asserted in the bank num-
ber field on the address bus. Unpredictable data is returned by the I/O
node.
As soon as the I/O node consumes the address and can free the buffer, it
issues a CSR write command to a TLWSDQRn register in local CSR broad-
cast space, where n is the physical node number of the I/O node. The I/O
node is then ready to receive another window space request. If the I/O
node acknowledges the CSR write command, it must also cycle the data
bus and provide data with good ECC. The data should be considered Un-