Figure 3. Representation of a bus clock signal
Over the years, some computer components have gained in speed more than others have. For this
reason, the components in a typical server are controlled by different clocks that run at different, but
related, speeds. These clocks are created by using various clock multiplier and divider circuits to
generate multiple signals based on the main system bus clock. For example, if the main system bus
operates at 100 MHz, a divider circuit can generate a PCI bus frequency of 33 MHz (system clock ÷
3) and a multiplier circuit can generate a processor frequency of 400 MHz (system clock x 4).
Computer components that operate in whole multiples of the system clock are termed synchronous
because they are "in sync" with the system clock.
Synchronous components operate more efficiently than components that are not synchronized
(asynchronous) with the system bus clock. With asynchronous components, either the rest of the
system or the component itself must wait one or more additional clock cycles for data or instructions
due to clock resynchronization. In contrast, synchronized components know on which clock cycle data
will be available, thus eliminating these timing delays.
Memory bus speed
The speed of the DRAM is not the same as the true speed (or frequency) of the overall memory
subsystem. The memory subsystem operates at the memory bus speed, which may not be the same
frequency (in MHz) as the main system bus clock. The two main factors that control the speed of the
memory subsystem are the memory timing and the maximum DRAM speed.
Burst mode access
The original DRAM took approximately six system bus clock cycles for each memory access. During
memory access, the RAS and CAS were sent first and then 64 bits of data were transferred through
the memory bus. The next sequential address access required a repeat of the RAS-CAS-Data
sequence. As a result, most of the overhead occurred while transferring row and column addresses,
rather than the data.
FPM and EDO improved performance by automatically retrieving data from sequential memory
locations on the assumption that they too will be requested. Using this process called burst mode
access, four consecutive 64-bit sections of memory are accessed, one after the other, based on the
address of the first section. So instead of taking six clock cycles to access each of the last three 64-bit
sections, it may take from one to three clock cycles each (see Figure 4).
Burst mode access timing is normally stated in the format "x-y-y-y" where "x" represents the number of
clock cycles to read/write the first 64 bits and "y" represents the number of clock cycles required for
the second, third, and fourth reads/writes. For example, prior to burst mode access, DRAM took up to
24 clock cycles (6-6-6-6) to access four 64-bit memory sections. With burst mode access, three
5
Печатать страницу