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1.2.2 Main Memory

Now that we have seen that information (data) can be represented in a computer using binary patterns, we can look at how this information is stored within the machine. An electronic circuit that can be switched ON or OFF can represent one binary digit or one bit of information. A class of such devices (called flip-flops) which can retain the value of a bit, even after the input to them changes (though only as long as power is applied to them), can be used to store a bit. The Main Memory block of Figure 1.1 is constructed of many of these devices, organized so that data (and instructions) may be stored there and subsequently accessed. Memory in present day computers is usually organized as a sequence of bytes (a byte is a group of eight bits). Each byte in memory is given a unique unsigned integer address, which may be considered its ``name''. (See Figure 1.2). A row of houses on a street with street addresses or a row of numbered mailboxes are reasonable analogies to memory addresses. The CPU (or any other device wishing to access memory) may place an address on a set of wires connected to the memory (called the address bus) in order to either read (load) or write (store) information in memory. Once information has been written to a particular location (address) in memory, it will remain unchanged unless a subsequent write is performed to the same address. Multiple bytes may be accessed (either simultaneously or sequentially) for data items large than a single byte. Like other information in the computer, an address is represented internally in binary. In the figure, we have shown the addresses both in binary and in hexadecimal form.

Computers are often classified by how many bits may be accessed simultaneously, e.g. 16 bits or 32 bits. The maximum number of bytes directly addressable in a computer depends on the number of bits in the memory address. A 16 bit machine usually allows 16 bits for address and a 32 bit machine usually allows anywhere form 17 to 32 bits for address. Since n bits can represent values, 16 bit addresses can address 64 KBytes (i.e. 65,536 bytes from byte addresses 0 to 65535) and 32 bit addresses can address 4 GigaBytes (over 4,000,000,000 bytes) directly.