Accelerated Graphics Port Interface Specification
AGP3.0 Interface Specification
Rev. 1.0
35
have at least 128 bytes of buffering. This is double that of AGP2.0 which requires a minimum of 64
bytes.
Subsequent transfers may be stalled on a block-by-block basis with the suppression of TRDY at the pre-
defined throttle point.
CLK
1 2 3 4 5 6 7 8 9
FRAME
AD
C/BE
IRDY
TRDY
WBF
AD_STBF
ADD
CMD
STOP
DEVSEL
1
st
Block (128B)
2
nd
Block (64B
)
WAIT
WAIT
Throttle
Point
Figure 2-8: Fast-Write Showing Wait State Insertion
2.3.3.3 Buffer-Full Flow Control for AGP Reads
The Buffer-Full scheme for stalling data returns for AGP reads uses a signal called RBF. The assertion
of RBF by the AGP Master is intended to prevent the start of a new data transfer transaction. However,
since there is a delay from the time of RBF assertion to the time the core-logic can take action, RBF may
not be able to stop the transactions (up to a maximum of two) that are already in the pipeline.
This happens if the first data transfer completes in one common clock cycle, and is followed by another
data transfer transaction in the immediately following cycle. If RBF is asserted in the same cycle as the
first transaction, it does not take effect until the third data transfer transaction. The amount of buffering
the AGP Master needs to support is dictated by the minimum and maximum request sizes. Table 14
illustrates the difference between AGP3.0 and AGP2.0 buffer requirements.
Table 14: Buffer Size Requirements Based on RBF Flow Control
Interface Type Min Transfer Size Max Transfer Size Min Buffering Needed
AGP2.0 =<16 bytes =>64 bytes 80 bytes
AGP2.0 =<16 bytes <64 bytes Max transfer size+16 bytes
AGP2.0 >16 bytes =>64 bytes 64 bytes
AGP2.0 >16 bytes <64 bytes Max transfer size
AGP3.0 =<32 bytes 64 bytes 96 bytes
AGP3.0 =<32 bytes <64 bytes Max transfer size + 32 bytes
AGP3.0 >32 bytes 64 bytes 64 bytes
AGP3.0 >32 bytes <64 bytes Max transfer size + 32 bytes
As with AGP, the use of RBF is discouraged since it negatively impacts the efficient use of the AGP3.0
interface. An AGP Master should, in general, not en-queue more read requests than the buffering
available to receive the requested data.