7.4 Read- Only Memory
When a computer program is loaded into the memory, it remains in the memory until it is overwritten or until the power is turned off. For some applications, the program never changes, and so it is hardwired into a read-only memory
(ROM). ROMs are used to store programs in videogames, calculators, micro- wave ovens, and automobile fuel injection controllers, among many other applications.
The ROM is a simple device. All that is needed is a decoder, some output lines, and a few logic gates. There is no need for flip-flops or capacitors. Figure 7-10
shows a four-word ROM that stores four four-bit words (0101, 1011, 1110, and 0000). Each address input (00, 01, 10, or 11) corresponds to a different stored word.
For high-volume applications, ROMs are factory-programmed. As an alternative, for low-volume or prototyping applications, programmable ROMs (PROMs) are often used, which allow their contents to be written by a user with a relatively inexpensive device called a PROM burner. Unfortunately for the early videogame industry, these PROM burners are also capable of reading the contents of a PROM, which can then be duplicated onto another PROM, or worse still, the contents can be deciphered through reverse engineering and then modified and written to a new, contraband game cartridge.
Although the PROM allows the designer to delay decisions about what information is stored, it can only be written once, or can be rewritten only if the existing pattern is a subset of the new pattern. Erasable PROMs (EPROMs) can be writ- ten several times, after being erased with ultraviolet light (for UVPROMs) through a window that is mounted on the integrated circuit package. Electrically erasable PROMs (EEPROMs) allow their contents to be rewritten electrically. Newer flash memories can be electrically rewritten tens of thousands of times, and are used extensively in digital video cameras, and for control programs in set-top cable television decoders, and other devices.
PROMs will be used later in the text for control units and for arithmetic logic units (ALUs). As an example of this type of application, consider creating an ALU that performs the four functions: Add, Subtract, Multiply, and Divide on eight-bit operands. We can generate a truth table that enumerates all 216 possible combinations of operands and all 22 combinations of functions, and send the truth table to a PROM burner which loads it into the PROM.
This brute force lookup table (LUT) approach is not as impractical as it may seem, and is actually used in a number of situations. The PROM does not have to be very big: there are 28 ´ 28 combinations of the two input operands, and there are 22 functions, so we need a total of 28 ´ 28 ´ 22 = 218 words in the PROM, which is small by current standards. The configuration for the PROM ALU is shown in Figure 7-11. The address lines are used for the operands and for the function select inputs, and the outputs are produced by simply recalling the precomputed word stored at the addressed location. This approach is typically faster than using a hardware implementation for the functions, but it is not extensible to large word widths without applying some form of decomposition.
32-bit operands are standard on computers today, and a corresponding PROM ALU would require 232 ´ 232 ´ 22 = 266 words which is prohibitively large.