Addressing is performed in two stages within a mainframe. The first stage involves the instruction address that resides in the instruction pointer register and the data address that is calculated from an address stored in a register, indexed, or included as part of the instruction word.
The second stage translates the addressed calculated in stage one into an actual, physical memory address for which the data is to be accessed from.
Mainframes originally required a standard keyboard as input and its output was onto a low resolution, mono-coloured text. However at present the PC’s that are connected to the mainframe have at least 256MB of RAM, and could have much more. Every system has cache memory and hard drives are of excess of 20Gigabytes plus.
The advantages of using a mainframe are they are extremely reliable. They can run efficiently and without problems for years. They also provide a medium for hundreds of users to work upon at once. They also allow users to work on the same task or different tasks at the same time. Mainframes also have a large storage space and are able to run thousands of instructions at once.
The disadvantages are that they are extremely expensive, with companies leasing them instead of buying them, as they come into the excess of over 7 figures sums. As explained above, they also require large office space to be housed. They also require regular air-conditioning to keep them at a maintainable level of temperature.
All these factors mean that Mainframes are an expensive option within the Computing world.
Midrange Computers
Midrange Computers are very popular within scientific communities for their efficiency and use of application within narrow scope. They are extremely useful for CAM (Computer Aided Manufacture), Internet Servers, data management and analytical processing.
Midrange Computers have an RISC (Reduced Instruction Set Computing) instruction set. This allows the Midrange computer to run reduced sets of instructions to run at a higher performance rate.
Midrange computers can be multi-user and multi-tasking.
The Midrange computers have a 32bit CPU and up to recently a 64bit CPU. The 32bit CPU has 32bit general purpose and 32bit addressing registers, with the 64bit CPU have 64bit registers respectively.
Programs written on each of the CPU architectures (32bit and 64bit) can be run on each CPU; however the converse may not be correct.
The clock speeds of the Midrange PC’s, mainly the PowerPC range from 100MHz to up to 1 GHz, which allows three or more instructions can be executed per cycle.
The PowerPC consists of 32bit, 64bit and occasionally 128bit registers. They provide large data execution, with the larger the data capacity, the greater speed it will produce.
The Addressing procedure in PowerPC’s is very interesting. Memory Addressing is only used for Load, Store and Branching instructions, with other instructions taking their operands from the registers and fields within the instruction itself.
It uses Logical addresses rather than Physical for storage within the computer, with four different translation techniques to convert logical addresses to physical addresses. These are Block Addressing, Disable Mode, Ordinary Translation and Direct Store Translation.
One of the advantages of a PowerPC over other forms of computing is the inbuilt Privilege system. This allows Supervisor access and regular user access to be able to use different functions. This allows security and protection for the system.
They are extremely efficient, fast and require less maintenance than Mainframes.
They are also able to provide management of networks of PC’s and terminals.
The disadvantage is that they are not as powerful as mainframes, which means a lot of companies are not interested in them.
Personal Computers
Personal Computers have overtaken the market with a storm over the past decade to the point where millions of homes have them all over the Globe. They are single user machines with the ability to be networked with other PC’s.
They currently run with a CPU speed of up to 3 GHz, which are made by the industry spear headers AMD and Pentium. The have an average RAM holding of 256 DDR RAM (Double Data Rate) and a typical Hard Drive space of 60GB. They also contain Cache Memory. This is a storage space that holds data to keep up to speed with the Processor speed.
Typical PC registers consist of eight 80bit floating point registers as well as General Purpose registers, in which addresses and data are held.
MMX (Multi Media Extension) and SIMD (Single Instruction Multiple Data) was then created by Pentium. This included new Microprocessor instructions to carry out larger instructions involving video, audio and graphical data, as well as being able to use one instruction to perform the same instruction on multiple data items. This was coupled with a large cache memory, which meant that lesser trips to the memory were required by the microprocessor. This gave Pentium up to 60% increase in performance over non-MMX machines.
It consists of seven different address modes, with Logical and Physical addressing taking place in the memory or address section.
The x86 architecture has been responsible for the PC as it is today. The instructions are the same as they were back in the 1980’s, with the exception that they have evolved into the form used today.
Virtual Storage also plays a large part on Personal Computers. Virtual Storage is the ability to be able to have a significantly larger memory capability than the real memory capacity. This allows larger programs and greater amounts of data to be processed than normal chips would allow.
The advantages of PC’s are that they can be extremely powerful and able to be used for a variety of different processes such as Accounting, Gaming and Word Processing.
The disadvantage of PC’s is that their capabilities are being updated extremely regularly. This means that a machine bought could be out of date within a year and can be expensive to upgrade. They are also fairly expensive, with an up-to-date machine costing over £1000 from new.
Palmtops
Palmtops or PDA’s (Personal Digital Assistant) are a useful way to carry around several tasks such as Address Book, organizer and phone book in one item. They are extremely mobile which appeals to business users.
They contain a Processor, such as the Motorola Dragonball, which is a lot cheaper and smaller than PC processors.
They run at around 20-75MHz, which is a lot slower than their PC counterpart who can run into Gigahertz, however a speed such as this is more than enough to run the programs a PDA contains. They contain up to 2MB of memory, with advanced models being able to be updated via Flash Memory to 32MB.
The PDA also contains an Operating System, but unlike the PC’s operating system the PDA’s Operating System only contains several predefined lines of code to instruct the Microprocessor.
As they only contain a small amount of instructions, they take up a small amount of memory (less than 100kb) so they can easily be stored on the PDA’s Read Only Memory.
The PDA also has no hard disk. Important programs such as the operating System, Address Book, Organizer etc are stored on the PDA’s ROM, while programs that the user wishes to install themselves are stored on the PDA’s RAM. They all use Solid-State memory, with some models of PDA opting for Flash Memory and Static RAM.
Input comes via a Keyboard or Touch Screen, with some using Hand Writing Recognition and several buttons to access applications.
The advantages of PDA’s are that they are extremely mobile. They allow several programs to be placed in one mobile item and are extremely good space savers.
They are extremely quick upon start up in which the programs held on ROM are immediately available.
The main disadvantage is that they come with a small amount of memory. They also require upgrading via Flash Memory, which is very expensive, if other programs are wished to be installed upon the PDA. They are also very expensive to buy, with a standard model such as the Palm Tungsten T2 costing £280, to a top of the range model such as COMPAQ IPAQ H3870 64MB costing £560.
Glossary
CISC (Complex Instruction Set Computer)
Computer designed with a full set of instructions that are intended to provide needed capabilities in the most efficient way.
RISC (Reduced Instruction Set Computer)
RISC enables the microprocessor to run a reduced set of instructions. RISC allows processing at a much higher speed.
Static RAM
SRAM (static RAM) is random access memory (RAM) that retains data bits in its memory as long as power is being supplied.
Flash Memory
Flash memory is a constantly powered type of memory that can be erased and reprogrammed in units of memory called blocks. Flash Memory is very popular in Digital Cameras and PDA’s to add extra storage capabilities.
MMX (Multi Media Extension)
MMX adds new Microprocessor instructions to deal with Media such as Video, Audio and Graphics. Allows MMX enabled Processors to run up to 60% more quickly.
SIMD (Single Instruction Multiple Data)
One of the three performance enhancements of MMX, SIMD lets one instruction within the microprocessor operate at the same time on multiple data items.
This is especially productive for applications involving Media such as Video and Audio.
FICON (Fiber Connectivity)
FICON is a high-speed input/output interface for mainframe computer connections to storage devices. FICON channels increase input/output capacity through the combination of a new architecture and faster physical link rates.
ESCON (Enterprise Systems Connection)
ESCON is a marketing name for a set of IBM and vendor products that interconnect S/390 computers with each other and with attached storage, attach local workstations with optical fibre technology.