The Memory

B. Memory
The memory of a computer system consist of tiny electronic switches, with each switch set in one of two states: open or close. It is however more convenient to think of these states as 0 and 1.

Thus each switch can represent a binary digit or bit, as it is known, the memory unit consists of millions of such bits, bits are organized into groups of eight bits called byte. Memory can be viewed as consisting of an ordered sequence of bytes. Each byte in this memory can be identified by its sequence number starting with 0, as shown in Figure 1. This is referred to as memory address of the byte. Such memory is called byte addressable memory.






Figure 1: Logical view of the system memory


8086 can address up to 1 MB (220 bytes) of main memory this magic number comes from the fact that the address bud of the 8086 has 20 address lines. This number is referred to as the Memory Address Space (MAS). The memory address space of a system is determined by the address bus width of the CPU used in the system. The actual memory in a system is always less than or equal to the MAS.
Two basic memory operations
The memory unit supports two fundamental operations: Read and
Write. The read operation read a previously stored data and the
write operation stores a value in memory. See Figure 2

Figure 2: Block diagram of system memory


Steps in a typical read cycle:
1- Place the address of the location to be read on the address bus.
2- Activate the memory read control signal on the control bus.
3- Wait for the memory to retrieve the data from the address memory location.
4- Read the data from the data bus.
5- Drop the memory read control signal to terminate the read cycle.

Steps in a typical write cycle:
1- Place the address of the location to be written on the address bus.
2- Place the data to be written on the data bus.
3- Activate the memory write control signal on the control bus.
4- Wait for the memory to store the data at the address location..
5- Drop the memory write control signal to terminate the write cycle.


Addresses:
group of bits which are arranged sequentially in memory, to enable direct access, a number called address is associated with each group. Addresses start at 0 and increase for successive groups. The term location refers to a group of bits witha unique address. Table 1 represents Bit, Byte, and Larger units.

Table1: Bit, Byte, and Larger units.

Name Number of Byte
Bit 0 or 1
Byte is a group of bits used to represent a character, typically 8-bit.
Word 2-bytes (16-bit) data item
Double Word 4-byte (32-bits)
Quadword 8-Bytes (64-bit)
Paragraph 16-bytes (128-bit)
KiloByte (KB) the number 210 = 1024 = 1K for KiloByte, (thus640K = 640 * 1024 = 655360 bytes)


Types of memory
The memory unit can be implemented using a variety of memory
chips- different speeds, different manufacturing technology, and
different sizes. The two basic types are RAM and ROM.

1- Read Only Memories (ROM):
ROMs allow only read operation to be performed. This memory is non-volatile. Most ROMs are programmed and cannot be altered. This type of ROM is cheaper to manufacture than other types of ROM. The program that controls the standard I/O functions (called BIOS) is kept in ROM, configuration software.








Other types of ROM include:
- Programmable ROM (PROM).
- Erasable PROM (EPROM) is read only memory that can be reprogrammed using special equipment.
- EAPROM, Electrically Alterable Programmable ROM is a Read Only Memory that is electrically reprogrammable.


2- Read/Write Memory
Read/Write memory is commonly referred to as Random Access Memory (RAM), it is divided into static and dynamic. Static RAM (SRAM): used for implementing CPU registers and cache memories.
Dynamic RAM (DRAM), the bulk of main memory in a typical computer system consists of dynamic ram. Dynamic RAM: main memory, or RAM is where program, data are kept when a program is running. It must be refreshed with in less than a millisecond or losses its contents.

Static RAM, used for special high speed memory called cache memory which greatly improves system performance. Static RAM keeps its value without having to be refreshed.