4 
as standard modules. Therefore, the detail designer has in most cases 
become a system designer. His task is to design a system that performs 
the specific functions he is concerned with, by using standard modules 
as system components whenever possible. 
STANDARD MODULES 
Any digital logic function can be implemented as a combination of 
a number of simple logic building blocks (e.g., NOR gates, or AND/OR 
gates). One may think about a standard module as a "black box" that 
has a set of inputs and outputs. The outputs are a function of the in 
puts, either in a fixed fashion (combinatorial logic), or in a variable 
way (sequential logic). A functioning operational system usually includes 
many such modules, which are selected, interconnected, and "programmed" 
by the designer to do the job. 
ROM (Read Only Memory) [6] 
These modules come in many forms, the primary variable being their 
capability to store bits of information. In all cases, the module has 
an address (input) that specifies which word is to be selected and pre 
sented as its output. The word, once it is put into its particular 
address location, cannot be changed or moved. Thus, the name Read Only 
Memory. 
ROM modules have found wide use in computers and calculators for 
storage of constants, function tables, and permanent computation se 
quences such as microprograms. However, it is obvious that ROM's can 
also be used to perform combinatorial logic functions; for a given input 
a specified output is obtained. ROM's are very fast. The access time 
to the stored data is normally less than 200 nanoseconds and can be con 
siderably less. 
PROM (Programmable Read Only Memory) [7] 
These modules are functionally similar to ROM modules, except that 
the contents of the memory can be erased and altered by using special 
techniques. Thus, PROM's are often used in prototype equipment when user 
requirements and other reasons (including design mistakes!) may still 
call for changes. 
RAM (Random Access Memory) 
These modules could be used to implement combinatorial logic.. How 
ever, such an application would be wasteful. The RAM modules have their 
own significant function: they store information in a fashion that per 
mits rapid alteration. RAM modules are widely used as normal random- 
access computer memory, in direct competition with magnetic core memories. 
However, fundamentally a rapid access storage of some kind (e.g., a register)