Intel 460GX manual Maximum Memory Configuration Using Two Cards

Memory Subsystem

Figure 5-1. Maximum Memory Configuration Using Two Cards

processor data bus

processor address bus

PB[71:0]

SDC

MDa[71:0]

MDb[71:0]

controls

MAb[24:0]

SAC

MDC

MDC

MDC

MDC

MAa[24:0]

bits

288 bits

MDC

MDC

MDC

MDC

x 72

x

72

x 72

x

72

x 72

x 72

x 72

x 72

MAC

288 bits

288 bits

row 1 of 4

row 1 of 4

x 72	x 72	x 72	x 72	MAC	x 72	x 72	x 72	x 72
						MAC
to PCI via expander bridge			Stack A					Stack B
row 4 of 4							row 4 of 4
Stack L						Stack R

memory card A

Each card is organized as 2 stacks of up to 4 rows each. A stack consists of 1 to 4 rows of DRAM which share a common data bus. A row consists of the 4 DIMM sockets which have a common address/control bus. A row is the minimum atomic unit that can be accessed. Each stack has a separate data path from the MDC to the DRAMs. Data may be transferring on both stacks simultaneously. For instance, on the same card, stack L could be doing a read while stack R is doing a write, or both could be doing a read.

Each row represents a set of memory devices simultaneously selected by a RAS signal and having a common address bus. Each row generates 288 bits (256 data, 32 ECC) of data. Each row uses 4 of the x72 (168 pin) DIMMs. There may be multiple banks per row. For instance, SDRAMs have 2, 4 or 8 banks internal to the chip. Or the DIMM may be organized such that there are 2 groups of memory on the DIMM itself (double-sided DIMMs), with 2 chips dotted together on the data pin. Since a row provides 256 bits (32 bytes) of data, the 64 byte line transfers of the processor will access a row twice to read or write the entire line. Data is interleaved by the MDCs to exchange 72 bits of data per transfer with the SDCs at the rate of one cache line every 30ns (2.13 GB/s) per interface.

There is a separate address and control bus for each memory port, with 1 card on each port. These are independent and may be driving addresses at the same time. While one bus is driving a read, the other could be driving a write from the write queue. This allows greater bandwidth and allows writes to be done without interfering with read traffic.

A system may be built with only one card, and provide only half the possible bandwidth, or have the memory on the motherboard itself. If there are 2 memory cards, then one is placed on each interface. Average memory latency is a function of how many interleaves are available and bandwidth. The present structure interleaves across cards first, then across stacks, and then rows. With 2 cards, each having 2 stacks of memory, Line 0 would be on cardB/stackR, Line 1 would be on cardA/stackR, Line 2 would be on card B/stackL, and Line 3 would be on cardA/stackL. See

5-2 Intel® 460GX Chipset Software Developer’s Manual