Chapter 4 System Support

4.3.1.1Data Request

Requesting data is accomplished in one of two ways; either multiplexed addressing (using the AD lines for addressing/data) or demultiplexed (“sideband”) addressing (using the SBA lines for addressing only and the AD lines for data only). Even though there are only eight SBA lines (as opposed to the 32 AD lines) sideband addressing maximizes efficiency and throughput by allowing the AD lines to be exclusively used for data transfers. Sideband addressing occurs at the same rate (1X, 2X, 4X, or 8X) as data transfers. The differences in rates will be discussed in the next section describing data transfers. Note also that sideband addressing is limited to 48 bits (address bits 48-63 are assumed zero). The IGP component supports both SBA and AD addressing, but the method and rate is selected by the AGP graphics adapter.

4.3.1.2Data Transfers

Data transfers use the AD lines and occur as the result of data requests described previously. Each transaction resulting from a request involves at least eight bytes, requiring the 32 AD lines to handle at least two transfers per request. The AGP v.2.0 specification (used on D315 models) supports three transfer rates: 1X, 2X, and 4X. The AGP v3.0 specification (used on d325 models) supports a fourth transfer rate, 8X. Regardless of the rate used, the speed of the bus clock is constant at 66 MHz. The following subsections describe how the use of additional strobe signals makes possible higher transfer rates.

AGP 1X Transfers

During a AGP 1X transfer the 66-MHz CLK signal is used to qualify the control and data signals. Each 4-byte data transfer is synchronous with one CLK cycle so it takes two CLK cycles for a minimum 8-byte transfer (Figure 4-5 shows two 8-byte transfers). The GNT- and TRDY- signals retain their traditional PCI functions. The ST0..3 signals are used for priority encoding, with “000” for low priority and “001” indicating high priority. The signal level for AGP 1X transfers may be 3.3 or 1.5 VDC.

T1

T2

T3

 

T4

T5

T6

T7

CLK

 

 

 

 

 

 

 

AD

 

D1A

D1B

D2A

 

D2B

 

GNT-

 

 

 

 

 

 

 

TRDY-

 

 

 

 

 

 

 

ST0..2

00x

xxx

xxx

xxx

 

xxx

xxx

Figure 4-5.AGP 1X Data Transfer (Peak Transfer Rate: 266 MB/s)

4-10Compaq D315 and hp d325 Personal Computers Featuring the AMD Athlon XP Processor

Second Edition – April 2003