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RAM

  RAM stands for random access memory. RAMs are volatile memory devices; they lose their data when power is removed.

Static RAMs (SRAM) store their data in arrays of flip flops. They retain their data as long as power is applied. SRAM is fast, and relatively expensive. PCs typically use SRAM as cache memory.

Dynamic RAM (DRAM) is a cheaper, slower memory technology which requires the data to be regularly refreshed. DRAM is widely used in PCs for bulk memory.

The SBC5206 microcomputer system has 1MB of DRAM (expandable), and the 5206 chip has facilities for DRAM control (see Section 10 of the Motorola MCF5206 User Manual). The 1MB RAM consists of a single $256K \times 32$ module (SIMM), and uses the address range $0000 0000 to $001F FFFF.

A simplified memory arrangement is shown in Figure 125. The address inputs together with chip select (active low, $\overline{CS}$) are used to select or address the memory device. When CS is not active, the data outputs are in their high impedance state. The write enable input (active low, $\overline{W}$) is connected to the processor's $R/\overline{W}$ pin and controls the data bus connection and enables writing to the memory device.

  

Figure 125: RAM module connected to microcomputer buses.

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