Unit - 3
CPU control unit design
The Control Unit is classified into two major categories:
- Hardwired Control
- Microprogrammed Control
Hardwired Control
The Hardwired Control organization involves the control logic to be implemented with gates, flip-flops, decoders, and other digital circuits.
The following image shows the block diagram of a Hardwired Control organization.
Fig 1 – Control unit of basic computer
- A Hard-wired Control consists of two decoders, a sequence counter, and several logic gates.
- An instruction fetched from the memory unit is placed in the instruction register (IR).
- The component of an instruction register includes; I bit, the operation code, and bits 0 through 11.
- The operation code in bits 12 through 14 are coded with a 3 x 8 decoder.
- The outputs of the decoder are designated by the symbols D0 through D7.
- The operation code at bit 15 is transferred to a flip-flop designated by the symbol I.
- The operation codes from Bits 0 through 11 are applied to the control logic gates.
- The Sequence counter (SC) can count in binary from 0 through 15.
Micro-programmed Control
The Microprogrammed Control organization is implemented by using the programming approach.
In Microprogrammed Control, the micro-operations are performed by executing a program consisting of micro-instructions.
The following image shows the block diagram of a Microprogrammed Control organization.
Fig 2 – Micro programmed control unit of a basic computer
- The Control memory address register specifies the address of the micro-instruction.
- The Control memory is assumed to be a ROM, within which all control information is permanently stored.
- The control register holds the microinstruction fetched from the memory.
- The micro-instruction contains a control word that specifies one or more micro-operations for the data processor.
- While the micro-operations are being executed, the next address is computed in the next address generator circuit and then transferred into the control address register to read the next microinstruction.
- The next address generator is often referred to as a micro-program sequencer, as it determines the address sequence that is read from control memory.
Key takeaway
The Hardwired Control organization involves the control logic to be implemented with gates, flip-flops, decoders, and other digital circuits.
The Microprogrammed Control organization is implemented by using the programming approach.
In Microprogrammed Control, the micro-operations are performed by executing a program consisting of micro-instructions.
Semiconductor memory is used in any electronics assembly that uses computer processing technology. Semiconductor memory is the essential electronics component needed for any computer-based PCB assembly.
In addition to this, memory cards have become commonplace items for temporarily storing data - everything from the portable flash memory cards used for transferring files, to semiconductor memory cards used in cameras, mobile phones, and the like.
The use of semiconductor memory has grown, and the size of these memory cards has increased as the need for larger and larger amounts of storage are needed.
To meet the growing needs for semiconductor memory, many types and technologies are used. As the demand grows new memory technologies are being introduced and the existing types and technologies are being further developed.
A variety of different memory technologies are available - each one suited to different applications. Names such as ROM, RAM, EPROM, EEPROM, Flash memory, DRAM, SRAM, SDRAM, as well as F-RAM and MRAM are available, and new types are being developed to enable improved performance.
Terms like DDR3, DDR4, DDR5, and many more are seen and these refer to different types of SDRAM semiconductor memory.
In addition to this the semiconductor devices are available in many forms - ICs for printed board assembly, USB memory cards, Compact Flash cards, SD memory cards, and even solid-state hard drives. Semiconductor memory is even incorporated into many microprocessor chips as on-board memory.
Fig 3 - Printed circuit board containing computer memory
Semiconductor memory: main types
There are two main types or categories that can be used for semiconductor technology. These memory types or categories differentiate the memory to how it operates:
RAM - Random Access Memory: As the names suggest, the RAM or random access memory is a form of semiconductor memory technology that is used for reading and writing data in any order - in other words as it is required by the processor. It is used for such applications as the computer or processor memory where variables and other stored and are required on a random basis. Data is stored and read many times to and from this type of memory.
Random-access memory is used in huge quantities in computer applications as current-day computing and processing technology requires large amounts of memory to enable them to handle the memory-hungry applications used today. Many types of RAM including SDRAM with its DDR3, DDR4, and soon DDR5 variants are used in huge quantities.
ROM - Read-Only Memory: A ROM is a form of semiconductor memory technology used where the data is written once and then not changed. Given this it is used where data needs to be stored permanently, even when the power is removed - many memory technologies lose the data once the power is removed.
As a result, this type of semiconductor memory technology is widely used for storing programs and data that must survive when a computer or processor is powered down. For example, the BIOS of a computer will be stored in ROM. As the name implies, data cannot be easily written to ROM. Depending on the technology used in the ROM, writing the data into the ROM initially may require special hardware. Although it is often possible to change the data, this gain requires special hardware to erase the data ready for new data to be written in.
As can be seen, these two types of memory are very different, and as a result, they are used in very different ways.
Each of the semiconductor memory technologies outlined below falls into one of these two types of categories. Each technology offers its advantages and is used in a particular way, or for a particular application.
Semiconductor memory technologies
There is a large variety of types of ROM and RAM that are available. Often the overall name for the memory technology includes the initials RAM or ROM and this gives a guide as to the overall type of format for the memory.
With technology moving forwards apace, not only are the established technologies moving forwards with SDRAM technology moving from DDR3 to DDR4 and then to DDR5, but Flash memory used in memory cards is also developing as are the other technologies.
In addition to this, new memory technologies are arriving on the scene and they are starting to make an impact in the market, enabling processor circuits to perform more effectively.
The different memory types or memory technologies are detailed below:
DRAM: Dynamic RAM is a form of random access memory. DRAM uses a capacitor to store each bit of data, and the level of charge on each capacitor determines whether that bit is a logical 1 or 0.
However these capacitors do not hold their charge indefinitely, and therefore the data needs to be refreshed periodically. As a result of this dynamic refreshing, it gains its name of being a dynamic RAM. DRAM is the form of semiconductor memory that is often used in equipment including personal computers and workstations where it forms the main RAM for the computer. The semiconductor devices are normally available as integrated circuits for use in PCB assembly in the form of surface mount devices or less frequently now as leaded components.
EEPROM: This is an Electrically Erasable Programmable Read-Only Memory. Data can be written to these semiconductor devices and it can be erased using an electrical voltage. This is typically applied to an erase pin on the chip. Like other types of PROM, EEPROM retains the contents of the memory even when the power is turned off. Also like other types of ROM, EEPROM is not as fast as RAM.
EPROM: This is an Erasable Programmable Read-Only Memory. These semiconductor devices can be programmed and then erased at a later time. This is normally achieved by exposing the semiconductor device itself to ultraviolet light. To enable this to happen there is a circular window in the package of the EPROM to enable the light to reach the silicon of the device. When the PROM is in use, this window is normally covered by a label, especially when the data may need to be preserved for an extended period.
The PROM stores its data as a charge on a capacitor. There is a charge storage capacitor for each cell and this can be read repeatedly as required. However, it is found that after many years the charge may leak away and the data may be lost.
Nevertheless, this type of semiconductor memory used to be widely used in applications where a form of ROM was required, but where the data needed to be changed periodically, as in a development environment, or where quantities were low.
Flash memory: Flash memory may be considered as a development of EEPROM technology. Data can be written to it and it can be erased, although only in blocks, but data can be read on an individual cell basis.
To erase and re-program areas of the chip, programming voltages at levels that are available within electronic equipment are used. It is also non-volatile, and this makes it particularly useful. As a result Flash memory is widely used in many applications including USB memory sticks, Compact Flash memory cards, SD memory cards, and also now solid-state hard drives for computers and many other applications.
F-RAM: Ferroelectric RAM is a random-access memory technology that has many similarities to the standard DRAM technology. The major difference is that it incorporates a ferroelectric layer instead of the more usual dielectric layer and this provides its non-volatile capability. As it offers a non-volatile capability, F-RAM is a direct competitor to Flash.
MRAM: This is Magneto-resistive RAM or Magnetic RAM. It is a non-volatile RAM technology that uses magnetic charges to store data instead of electric charges.
Unlike technologies including DRAM, which requires a constant flow of electricity to maintain the integrity of the data, MRAM retains data even when the power is removed. An additional advantage is that it only requires low power for active operation. As a result, this technology could become a major player in the electronics industry now that production processes have been developed to enable it to be produced.
P-RAM / PCM: This type of semiconductor memory is known as Phase-change Random Access Memory, P-RAM, or just Phase Change memory, PCM. It is based around a phenomenon where a form of chalcogenide glass changes its state or phase between an amorphous state (high resistance) and a polycrystalline state (low resistance). It is possible to detect the state of an individual cell and hence use this for data storage. Currently, this type of memory has not been widely commercialized, but it is expected to be a competitor for flash memory.
PROM: This stands for Programmable Read-Only Memory. It is a semiconductor memory that can only have data written to it once - the data written to it is permanent. These memories are bought in a blank format and they are programmed using a special PROM programmer.
Typically a PROM will consist of an array of fusible links some of which are "blown" during the programming process to provide the required data pattern.
SDRAM: Synchronous DRAM. This form of semiconductor memory can run at faster speeds than conventional DRAM. It is synchronized to the clock of the processor and is capable of keeping two sets of memory addresses open simultaneously. By transferring data alternately from one set of addresses, and then the other, SDRAM cuts down on the delays associated with non-synchronous RAM, which must close one address bank before opening the next.
Within the SDRAM family, there are several types of memory technologies that are seen. These are referred to by the letters DDR - Double Data Rate. DDR4 is currently the latest technology, but this is soon to be followed by DDR5 which will offer some significant performance improvements.
SRAM: Static Random Access Memory. This form of semiconductor memory gains its name from the fact that, unlike DRAM, the data does not need to be refreshed dynamically.
These semiconductor devices can support faster read and write times than DRAM (typically 10 ns against 60 ns for DRAM), and besides its cycle time is much shorter because it does not need to pause between accesses. However they consume more power, are less dense, and more expensive than DRAM. As a result of this SRAM is normally used for caches, while DRAM is used as the main semiconductor memory technology.
Semiconductor memory technology is developing at a fast rate to meet the ever-growing needs of the electronics industry. Not only are the existing technologies themselves being developed, but considerable amounts of research are being invested in new types of semiconductor memory technology.
In terms of the memory technologies currently in use, SDRAM versions like DDR4 are being further developed to provide DDR5 which will offer significant performance improvements. In time, DDR5 will be developed to provide the next generation of SDRAM.
Other forms of memory are seen around the home in the form of USB memory sticks, Compact Flash, CF cards, or SD memory cards for cameras and other applications as well as solid-state hard drives for computers.
The semiconductor devices are available in a wide range of formats to meet the differing PCB assembly and other needs.
Memory Organisation
The memory is organized in the form of a cell, each cell can be identified with a unique number called to address. Each cell can recognize control signals such as “read” and “write”, generated by the CPU when it wants to read or write an address. Whenever the CPU executes the program there is a need to transfer the instruction from the memory to the CPU because the program is available in memory. To access the instruction CPU generates the memory request.
Memory Request:
Memory request contains the address along with the control signals. For Example, When inserting data into the stack, each block consumes memory (RAM) and the number of memory cells can be determined by the capacity of a memory chip.
Example: Find the total number of cells in a 64k*8 memory chip.
Size of each cell = 8
Number of bytes in 64k = (2^6)*(2^10)
Therefore,
The total number of cells = 2^16 cells
With the number of cells, the number of address lines required to enable one cell can be determined.
Word Size:
It is the maximum number of bits that a CPU can process at a time and it depends upon the processor. Word size is a fixed size piece of data handled as a unit by the instruction set or the hardware of a processor.
Fig 4 - Example
Word size varies as per the processor architectures because of generation and the present technology, it could be low as 4-bits or high as 64-bits depending on what a particular processor can handle. Word size is used for several concepts like Addresses, Registers, Fixed-point numbers, Floating point numbers.
Key takeaway
Semiconductor memory is used in any electronics assembly that uses computer processing technology. Semiconductor memory is the essential electronics component needed for any computer-based PCB assembly.
In addition to this, memory cards have become commonplace items for temporarily storing data - everything from the portable flash memory cards used for transferring files, to semiconductor memory cards used in cameras, mobile phones, and the like.
References:
1. “Computer Organization and Design: The Hardware/Software Interface”, 5th Edition by David A. Patterson and John L. Hennessy, Elsevier.
2. “Computer Organization and Embedded Systems”, 6th Edition by Carl Hamacher, McGraw Hill Higher Education.
3. “Computer Architecture and Organization”, 3rd Edition by John P. Hayes, WCB/McGraw-Hill
4. “Computer Organization and Architecture: Designing for Performance”, 10th Edition by William Stallings, Pearson Education.
5. “Computer System Design and Architecture”, 2nd Edition by Vincent P. Heuring and Harry F. Jordan, Pearson Education.
