Random Access Memory

Random Access Memory (RAM): The part of the computer that stores instructions to be performed, and data that are to be processed. Here we will take a look at how RAM interacts with the rest of the computer hardware and software.

The CPU's central processor is designed that it will only execute a sequence of program instructions if they are stored in memory it is directly connected to. Memory is a circuit capable of storing a list of numbers, and may be though of a number warehouse. Imagine a warehouse full of small pigeon holes. Each one capable of storing a number (called a byte) and with a unique address, the first being address 0, then 1, and so on... The processor may say "put this number in address 345", or "what number is in address 223?"

Memory uses microscopic switches to represent numbers in base 2 (binary). One byte contains 8 switches (bits) capable of storing any whole number between 0 and 255.

Each byte is made up of eight switches.

PC essentials: Part 3: the motherboard 1994, Oct

To illustrate the importance of RAM, consider the situation of Starting up your PC and loading in an application.

1. PC is turned on

2. The Control Unit accesses ROM for startup instructions; eg. Checking the Base Hardware components.

           +-----+
           |     |       +[A:]----+     +[C:]--------+
 +-----+   |     |       |        |     |            |
 |     |   |     |       |        |     |            |
 | ROM |   | RAM |       | Floppy |     | Hard Disk  |
 +-----+   +-----+       +--------+     +------------+

3. A ROM instruction then attempts to access a floppy drive (Drive A) -- note on your PC the diskette light just after power on.

           +-----+
           |     |       +[A:]----+     +[C:]--------+
 +-----+   |     |       |        |     |            |
 |     |   |     |       |        |     |            |
 |     |   |     |       |        |     |            |
 +-----+   +-----+       +--------+     +------------+
 | ROM |   | RAM |       | Floppy |     | Hard Disk  |

4. If there is no SYSTEM disk in drive A, a Hard Drive is sought (Drive C).

           +-----+
           |     |       +[A:]----+     +[C:]--------+
 +-----+   |     |       |        |     |            |
 |     |   |     |       |        |     |            |
 |     |   |     |       |        |     |Command.com |
 +-----+   +-----+       +--------+     +------------+
 | ROM |   | RAM |       | Floppy |     | Hard Disk  |

5. The System Software is located (eg. for MS-DOS; COMMAND.COM) and loaded into RAM

           +-----+
           |     |       +[A:]----+     +[C:]--------+
 +-----+   |     |       |        |     |            |
 |     |   |     |       |        |     |            |
 |     |   |     |       |        |     |Command.com |
 +-----+   +-----+       +--------+     +------------+
 | ROM |   | RAM |       | Floppy |     | Hard Disk  |

6. The instructions for accessing files from the disk; such as, COPY, DEL, DIR are contained in RAM (in Command.COM).

          +-----------+
          |           |  +[A:]----+  +[C:]--------+
 +-----+  |           |  |        |  |            |
 |     |  |           |  |        |  |            |
 |     |  |Command.com|  |        |  |Command.com |
 +-----+  +-----------+  +--------+  +------------+
 | ROM |  | RAM       |  | Floppy |  | Hard Disk  |
7. An application may be loaded; we will consider two cases; 

• A Standard DOS application (Lotus 123)
• Windows applications

Eg. Lotus 123

Type "123" or "LOTUS" at the DOS prompt, and the instructions (Software) are loaded into RAM. Finally your spreadsheet is stored in RAM until saved to a Disk.

RAM - Lotus

+------+-------------------------+
|      | Free Memory             |
|  :   | :                       |
|  :   | :                       |
|  :   | Spreadsheet Document(s) |
|  :   | Lotus123   (123.COM)    |
|  0kb | MS-DOS Operating System |
+------+-------------------------+

Eg. Windows;

Type "WIN" at the DOS prompt and the instructions stored in WIN.EXE are loaded into RAM. As windows applications are loaded, they are also loaded into RAM, along with their respective data files.

RAM - Windows

+------+-------------------------+
|      | Free Memory             |
|  :   | :                       |
|  :   | :                       |
|  :   | Paint Document          |
|  :   | Windows Paintbrush      |
|  :   | WP Document(s)          |
|  :   | Windows Word Processor  |
|  :   | Windows   (WIN.EXE)     |
|  0kb | MS-DOS Operating System |
+------+-------------------------+

How do we access a particular memory location to;

• add to,
• change, or
• delete the data in it?

Memory available for the computer user can be accessed via

• an Application Program such as Lotus 123, or
• a programming language such as BASIC, COBOL

With an application program, you do not have direct control over where the data is stored in memory, with a programming language you can exert more control over where data is stored.

Operating Systems such as OS/2 will not allow direct memory addressing, as Virtual Storage is used, where both the Hard Disk and RAM interact as if they we one unit..

• MS-DOS

	Extended Memory: Memory above 1Mb in 286 and later PC's. Can only be used by special protected mode programs such as Windows 3.0. Standard DOS Programs cannot directly access extended memory, though it can store RAM drives, disk caches and for creating HMA
	High Memory Area (HMA) an extra 64Kb "bank" of memory DOS can access on systems with an Extended Memory Specification driver. The HMA can be controlled by only one program. eg. Windows or a Network driver. By placing an application into HMA, you save conventional memory.
	Upper Memory Blocks (UMB) The area in memory in DOS based PC's from 640Kb to 1Mb. This was originally set aside for future expansion, video memory, adaptor cards and additional BIOS. As PC's evolved little was used. This allowed UMBs to be created, and Temporary Stay Resident (TSR) programs to be loaded into the UMB's to free up conventional memory.
	Conventional Memory The lower 640KB of memory in which DOS runs, loads applications and where programs manipulate their data.

• Types
  • Dynamic random access memory (Dram):
    Basic, no frills RAM chips. Require regular refreshing for the RAM to keep it's contents.
    • Fast page mode RAM (FPM Ram):
      With 486 or older Pentium. Typically 1-16MB density
    • Extended data out RAM (EDO Ram):
      Designed to keep data available to your processor for an extended amount of time by holding the last data request in its buffer, while at the same time retrieving the next request. Typically 4-16MB density.
      (Imagine you need to move a stack of papers from a desk on your left to one on your right and can only use one hand to do it that's standard Dram. EDO Ram allows you to transfer the paper from your first hand to the second hand, while your first hand goes back to retrieve more paper from the original stack.)
    • Synchronous dynamic random access memory (SDRAM):.
  • Static random access memory (SRAM):
    Does not require constant refreshing so is faster (and more expensive) and is used primarily for level 2 cache.
  • Error checking and correcting Ram (ECC Ram):
    Targeted at high end machines such as file servers. The data checks itself before it reaches the motherboard and corrects any errors it finds.

(Wright, H. 1996, Aug) & (Santoni A, 1996 Oct)

RAM Socket: A small card where Memory chips are mounted onto. Either Single in-line memory module (SIMM):. with either 30 or 72 pins or Dual in-line memory module (DIMM):. with 168 pins. RAM Speed: Memory chips are designed to operate at certain maximum speeds, typically 60,70 or 80 nanoseconds (billionths of a second). RAM Parity: Memory modules are either parity or non-parity. A parity module has an extra chip (usually a ninth) for error checking. Some motherboards work with either, but they cannot be mixed. RAM Size/sockets: Simm sockets are usually arrranged in pairs. Both sockets in a single bank must be filled with Simms of identical memory size. (Steers, K. 1997, Sep)

RAM Disk: A portion of RAM is reserved and made to simulate a Disk Drive.

[Rev: 23/8/98] Jun-97 © 1997-98 V/2-Com (Verhaart), P O Box 8415, Havelock North, New Zealand.