MICROPROCESSORS: A BRAIN TO THE HARDWARE

 

I. The microprocessor forms the heart of a microcomputer.

The first microprocessors were developed in 1971 as an off­shoot¹ of pocket calculator development. Since then there has been a tremendous upsurge² of work in this field and some years later there appeared dozens³ of different microprocessors commercially available.

The age of the microprocessor is not great. Yet, we have seen the evolution of the microprocessor as it progressed from early applications in simple hand-held calculators through 4 and 8-bit controller applications towards more sophisticated processing operations.

Microprocessors are used primarily to replace or upgrade⁴ random⁵ logic design.

By taking advantage of the knowledge and concepts gained in mainframe and minicomputer applications better and more so­phisticated microprocessors are beginning to emerge. What we see are: larger and denser chips; higher resolution; higher speed; specially designed RAMs (random access⁶ memory) and ROMs (read-only memory); specially designed I/O and peripheral⁷ in­terface circuits; on-chips clock and timing circuits; more extensive and more powerful instruction sets⁸ and lower power dissipa­tion.⁹

With the enormous efforts now directed to MPs, performance will improve rapidly. A far larger number of bits (higher resolu­tion), higher speeds, more extensive and more powerful instruc­tion sets, and elimination¹⁰ of non-LSI components have come. In addition, software for these machine would also evolve into more t standardized forms.

Microprocessors are now appearing in many types of equip­ment and their field of application will inevitably¹¹ widen.

Since these devices are likely to be used by the million in the near future, it is reasonable to ask what a microprocessor is, how it can boused and what its future mipalft¹² will be.

As mentioned before computer actually refers to a computing system including hardware (processor, I/O circuits, power sup­plies,¹³ control panel, etc.) and software (instruction manual, user's manual, assembler, and diagnostic and service routines). Processor is known to refer to the processing circuits: central pro­cessing unit, memory, interrupt unit, clock, and timing.¹⁴ Most processors also include computer software.

Central processing unit (CPU) —heart of the proces­sor — consists of the register array, arithmetic and logic unit, con­trol unit (including micro-ROM), and bus¹⁵ control circuits. Micro software may also include: microinstruction manual, micro assembler, etc.

Mini - has been used with computers and refers to the sys­tems having mainframe only, no peripherals.

Micro —can refer to computers, processors, or processing units. Smaller size and lower cost are usually obtained through use of LSI circuits.

Monolithic — generally implies¹⁶ a single block or chip of sili­con. A monolithic CPU is therefore a single-chip CPU, produced with LSI techniques. The term monolithic processor eliminates the need to differentiate¹⁷ between mini and micro. The Acronym MP can represent either micro or monolithic processor.

Any processing unit has a logic and a control unit. Broadly speaking, a control system can be defined as an element or series of elements that implement the transformation of a physical input excitation¹⁸ into a corresponding¹⁹ physical output response in some deterministic manner. The logic element is an mtegral part of any control system. The logic element is known to be the basic component of all computers. A great deal of effort has been directed towards reducing the size of the basic logic element.

The very first microprocessors were fabricated using PMOS technology. These were, however, relatively slow devices princi­pally because "holes" in the p -type material have a low mobility. Later, improved technology permitted microprocessors to be con­structed using n -type MOS and these microprocessors are almost as fast as normal minicomputers with speeds of three or four mi­croseconds per instruction. Some microprocessors are now made using CMOS. The speed and logic aensity of CMOS are inferior²⁰ to n-typa MOS but the process does have some significant advantages. First of all, it has a low power consumption since power is only consumed when a logic element changes a state. Secondly, it can operate over a wide voltage гаngе.²¹ As а result, electronics based on CMOS can operate successfully with "noisy" power supplies and the low consumption makes it quite feasible²² to use a simple battery to maintain the security²³ of supply for several weeks. This type of microprocessor has clear advantages over the other types if it is intended for use in exacting²⁴ or inaccessible environments. Further development should improve the logic density of CMOS and it is likely to become a dominant technology in the microprocessor field.

The only cloud on the CMOS horizon comes from a new de­velopment of the normal bipolar circuit. A new semiconductor configuration called integrated injection logic (IIL) has been de­vised²⁵ which eliminates the need for any resistors, capacitors or transistor isolation. This enables an extremely compact logic circuit to be formed which has a low power consumption while maintainin thе normal speed of transistor-transistor logic (TTL).

The bulk of present-day microprocessor and memory logic is implemented using PMOS and NMOS processes, since these pro­cesses are now well developed and offer good logic density. In the future IIL and CMOS are likely to become the most popular types, and the general trends in technology indicate that lower power consumption, higher speeds and improved logic densities can be confidently anticipated.²⁶

The key features to consider in any microprocessor are: word²⁷ length; architecture; speed; programming flexibility, etc.

Word length should be the first feature to consider.

The processor handles binary data in the form of "words". A word is a set of binary bits which is used to represent a binary number within the computer. It is the number of bits in the com­puter "word" which limits the numerical range of the data that the processor can handle. Microprocessors are structured for fixed word length or for modular expansion by a parallel combination of building-block chips.

The versatility of the microprocessor has altered the entire ar­chitecture of modem computer systems. No longer²⁸ is the pro­cessing of information carried out only in the computer's central processing unit. Today there is a trend towards distributing more processing capability throughout a computer system, with various areas. For example, an input-output port may have a controller to regulate the flow of information through it. At times the controller may accept commands from the CPU and send signals back in or­der to coordinate its operations with those of the rest²⁹ of the system; at other times the controller may operate independently of the CPU.

II. Distributing microprocessing is a technique in which the main microprocessor of the PC directs other microprocessors throughout the PC system to perform specific functions for it and report their status.

New forms of I/O are also acquiring³⁰ sophisticated ca­pabilities with distributed microprocessing. These "intelligent" I/O modules perform some of the calculations formerly done by the main microprocessor, store information temporarily,³¹ and do other functions under the direction of the main microprocessor.

Some remote I/O modules have microprocessors resident³² in the modules. Remote I/O modules use the resident microproces­sors to shorten the effective scan time. However, with indepen­dent intelligence³³ in the I/O, if something happens to the PC, the I/O module might already have acted on misinformation. Hence, I/O modules with a resident microprocessor should include appropriate³⁴ instructions for fail-safe shutdown³⁵ should the PC develop a fault.³⁶

A trend that is beginning to emerge in microprocessor design is the incorporation or troubleshooting³⁷ aids heretofore (дo сих пор) available only on larger computers.

Provisions³⁸ can and are being made in the architecture. Whereas early developments were concerned with imple­mentation of simple architectures with fundamental concepts and operations, the technology has now advanced to the point where significantly more sophisticated hardware can be (and is being) implemented in current and future microprocessor generations. For example, some relatively new functions available in today's PC's may include: Moving blocks of data from memory location to memory location or from I/O location to memory location with a single instruction; Matrix operations such as logical AND and logical OR for comparing on/off bit patterns; Expanded mathe­matical abilities. Most PCs have double precision arithmetic.

The ease or difficulty with which each element can communi­cate with another will affect how much the data are manipulated before they are transmitted through the network. The major ob­stacle to designing an effective distributed-processing system is the difficulty involved in writing the system's software, which must enable the various elements of the network to operate and interact efficiently.

There is a crucial³⁹ need for easy methods of documenting programs and changes made to them.

Programmability—that flexible feature not found in random-logic designs — can be obtained in microprocessors on one of two levels. A very detailed level of control is provided at the micro-in­struction level. These micro-instructions may be used to obtain a macro, or machine-language, instruction set, which is then used to write control programs for microprocessor. New machine-lan­guage instructions may be defined by coding new microroutines. In this way an instruction set can be tailored to an application. Control programs can also be written in microcode. This provides increased execution speed and more detailed control at the ex­pense of more difficult programming. Microprocessors that are not microprogrammable contain fixed, general-purpose instruc­tion sets, that are often adequate⁴⁰ for most applications.

Users have long felt a need to have a means of automatically adding comments and explanations to a hard copy of user pro­gram.'With the high-level language's code format and program­ming capabilities, this need is reaching a critical point.

The use of microprocessors makes systems easier operate and maintain. Microprocessors provide greater application flexibility. Today microprocessors are designed with communications in mind so it is possible to link these processors together in a net­work. It is attractive for a number of reasons.

We can look forward to even more sophisticated system func­tions including digital to analog conversion⁴¹ and vice versa, more arithmetic capability such as matrix inversion, etc., and massive amounts of memory.