Compaq W4000 manual Data Request, Data Transfers, CLK D1A D1B D2A D2B GNT Trdy

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, or 4X) 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 MCH 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 82845 MCH supports three transfer rates: 1X, 2X, and 4X. 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)

4-12Compaq Evo and Workstation Personal Computers

Featuring the Intel Pentium 4 Processor

Second Edition – January 2003