Chapter 6-01

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6.1 Storage Technologies

6.1.1 Random-Access Memory

Random-access memory (RAM) comes in two varieties—static and dynamic. Static RAM (SRAM) is faster and more expensive than Dynamic RAM (DRAM). SRAM is used for cache memories, both on and off the CPU chip. DRAM is used for the main memory plus the frame buffer of a graphics system. Typically, a desktop system will have no more than a few megabytes of SRAM, but hundreds or thousands of megabytes of DRAM.

Static RAM

SRAM stores each bit in a bistable memory cell. Each cell is implemented with a six-transistor circuit. This circuit has the property that it can stay indefinitely in either of two different voltage configurations, or states. Any other state will be unstable—starting from there, the circuit will quickly move toward one of the stable states. Such a memory cell is analogous to the inverted pendulum illustrated in Figure 6.1.

Due to its bistable nature, an SRAM memory cell will retain its value indefinitely, as long as it is kept powered. Even when a disturbance, the circuit will return to the stable value when the disturbance is removed.

Dynamic RAM

DRAM stores each bit as charge on a capacitor which is very small. Unlike SRAM, a DRAM memory cell is very sensitive to any disturbance. When the capacitor voltage is disturbed, it will never recover.

Various sources of leakage current cause a DRAM cell to lose its charge within a time period of around 10 to 100 milliseconds. The memory system must periodically refresh every bit of memory by reading it out and then rewriting it. Some systems also use error-correcting codes, where the computer words are encoded a few more bits (e.g., a 32-bit word might be encoded using 38 bits), such that circuitry can detect and correct any single erroneous bit within a word.

The cells (bits) in a DRAM chip are partitioned into d supercells, each consisting of w DRAM cells. A d × w DRAM stores a total of d*w bits of information. The supercells are organized as a rectangular array with r rows and c columns, where r*c = d. Each supercell has an address of the form (i, j), where i denotes the row, and j denotes the column.

Information flows in and out of the chip via external connectors called pins. Each pin carries a 1-bit signal.

Figure 6.3 shows two of these sets of pins: eight data pins that can transfer 1 byte in or out of the chip, and two addr pins that carry two-bit row and column supercell addresses. Other pins that carry control information are not shown.

Each DRAM chip is connected to some circuitry, known as the memory controller, that can transfer w bits at a time to and from each DRAM chip. To read the contents of supercell (i, j), the memory controller sends the row address i and the column address j to the DRAM. The DRAM responds by sending the contents of supercell (i, j ) back to the controller. The row address i is called a RAS (Row Access Strobe) request. The column address j is called a CAS (Column Access Strobe) request. Notice that the RAS and CAS requests share the same DRAM address pins.

For example, to read supercell (2, 1) from the 16 × 8 DRAM in Figure 6.3, the memory controller sends row address 2, as shown in Figure 6.4(a). The DRAM responds by copying the entire contents of row 2 into an internal row buffer. Next, the memory controller sends column address 1, as shown in Figure 6.4(b). The DRAM responds by copying the 8 bits in supercell (2, 1) from the row buffer and sending them to the memory controller.

DRAM chips are packaged in memory modules that plug into expansion slots on the main system board (motherboard). Common packages include the 168 pin dual inline memory module (DIMM), which transfers data to and from the memory controller in 64-bit chunks, and the 72-pin single inline memory module (SIMM), which transfers data in 32-bit chunks.

Figure 6.5 shows the basic idea of a memory module. The example module stores a total of 64 MB (megabytes) using eight 64-Mbit 8M × 8 DRAM chips, numbered 0 to 7. Each supercell stores 1 byte of main memory, and each 64 bit doubleword at byte address A in main memory is represented by the eight supercells whose corresponding supercell address is (i, j ). In the example in Figure 6.5, DRAM 0 stores the first (lower-order) byte, DRAM 1 stores the next byte, and so on.

To retrieve a 64-bit doubleword at memory address A, the memory controller converts A to a supercell address (i, j ) and sends it to the memory module, which then broadcasts i and j to each DRAM. In response, each DRAM outputs the 8 bit contents of its (i, j ) supercell. Circuitry in the module collects these outputs and forms them into a 64-bit doubleword, which it returns to the memory controller.

Main memory can be aggregated by connecting multiple memory modules to the memory controller. In this case, when the controller receives an address A, the controller selects the module k that contains A, converts A to its (i, j ) form, and sends (i, j ) to module k.

DRAMs and SRAMs are volatile that they lose their information if the supply voltage is turned off. Nonvolatile memories retain their information even when they are powered off. Nonvolatile memories are referred to as read-only memories (ROMs), even though some types of ROMs can be written to as well as read. ROMs are distinguished by the number of times they can be reprogrammed (written to) and by the mechanism for reprogramming them.

Flash memory is a type of nonvolatile memory. Flash memories are everywhere, providing fast and durable nonvolatile storage for many electronic devices.

Programs stored in ROM devices are often referred to as firmware. When a computer system is powered up, it runs firmware stored in a ROM. Some systems provide a small set of primitive input and output functions in firmware, for example, a PC’s BIOS (basic input/output system) routines.

Data flows back and forth between the processor and the DRAM main memory over shared electrical conduits called buses. Each transfer of data between the CPU and memory is accomplished with a series of steps called a bus transaction. A read transaction transfers data from the main memory to the CPU. A write transaction transfers data from the CPU to the main memory.

A bus is a collection of parallel wires that carry address, data, and control signals. Depending on the particular bus design, data and address signals can share the same set of wires, or they can use different sets. Also, more than two devices can share the same bus. The control wires carry signals that synchronize the transaction and identify what kind of transaction is currently being performed.

The I/O bridge translates the electrical signals of the system bus into the electrical signals of the memory bus. The I/O bridge also connects the system bus and memory bus to an I/O bus that is shared by I/O devices such as disks and graphics cards.

6.1.2 Disk Storage

6.1.3 Solid State Disks

6.1.4 Storage Technology Trends

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