3.3.3. Address Field of the Computer (Map)

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To make the computer processed data in binary principle of bits in the data should not be greater than the capacity of the register in the microprocessor. Capacity registry microprocessor is usually in accordance with size of the data bus to a computer, although there are versions where the capacity of multiple register data bus size, and is then filled into two bus clock readings.


Example III

How big a decade integer number can be stored in the register of microprocessor capacity:

            a.) 4 bit
            b.) 8 bit
            c.) 16 bit
            d.) 32 bit

Analogous to the previous capacities are as follows:

a.) 11112 = F16 = 1510

can store numbers from 0 to 15, or up to 16 different integer value.

b.) 1111 11112 = FF16 = 25510

can store numbers from 0 to 255, or up to 256 different integer value.

c.) 1111 1111 1111 11112 = FFFF16 = 65'53510

d.) 1111 1111 1111 1111 1111 1111 1111 11112 = FFFF FFFF16 = 4'294'967'29510

Numbers that are greater than the capacity of computer registry handles in exponential form.

To make the data stored in computer memory, and later be retrieved, it is necessary to 'store' to a specific memory location, called a MEMORY LOCATION, and facilitate its rapid retrieval. Size of the memory locations in accordance with the number of bits data bus, which means that the data is greater than the capacity of one memory location must be stored in multiple memory locations. Electronic circuit, which is the basis of each individual memory bits in this location, called the MEMORY ELEMENT or MEMORY CELLS.

As for this memory is usually organized as a chessboard or a crossword puzzle in which in every field there is a memory location that represents a group of cells.

 Memory array
Figure 3.3.6 Memory array of 8 bit computer.

Each memory location determined by the address that contains location location axis 'R' axis and 'S'. According to this Figure constituted to address 16 bit binary number to the first eight bits define a line, and the other eight bits column. Microprocessor be obtained on the basis of instructions, first determine the address line and then the column and did retrieve data. So, to access the data we need to perform three operations, three bars. Data is stored in memory locations size of 8 bits (1 byte). Address is determined by the data on the address bus, the first row (R) and then the column (S), and the data is transmitted to the data bus. When will read individual row and column addresses and data read and transfer data on the bus define orders sent via the control bus.

Special electronic circuits, decoders addresses, based on reading the binary number of the address register address signal voltage on one of its output lines in which the row and column where the desired memory location. So, for the organization of the image of the needy are two decoders. At the point where both the voltage is present, where the 'cross' (logical AND), it is possible to 'read' content sites. Following the example of the image of both the decoder had to FFH (256) lines for the choice of location.

Overall 'crossword' called ADDRESS FIELD of MEMORY. The total number of memory locations multiplied by the number of bytes in a location in memory CAPACITY. Specifically, a computer with a larger data bus of the examples in the picture will have a location of size 2, 4 or 8 bytes, or even more, depending on the construction of a computer. As for this, it follows that memory of which has three dimensions, two describe the place where the information is stored (location), and a third the size of storage (location).

Some memory locations are unavailable during the work to be written, but only for reading. These are locations where the bootstrap program locates the address of the I / O channels and the BIOS software routines and calls.

The arrays in memory constitutes MEMORY MAP computers. It describes the locations of which are single ROM contents, location will serve as a buffer for communication via the I / O channel, which locations are available for use, which will be used for load control and the like.


Example IV

Calculate the capacity of computer memory with an 8-bit bus term data and 16 bit address bus of the example in the picture.

One address bar describes the 00H and FFH last, and are determined by the first eight bits addresses, or 256 locations. The same as there are columns with certain other eight bits addresses.

Total number of memory locations is:

.L = 1111 11112  1111 11112 = FF16  FF16 = 256  256 = 65'536 locations (data)

In one location, composed of 8 cells fit 8 bits, the data size of 1 B.

Memory capacity is:

.C = L  size of location = 65'536 locations  1 B / location = 65'536 B

Such a unit of measurement is quite impractical, and would rather use higher; KILOBYTE.

.1 kB = 210 B = 1024 B

Thus, the memory capacity of 8 bit computers is:

.C = 65'536 B / 1024 = 64 kB

Larger units of kB were Megabyte, Gigabyte and furthermore, that:

1 MB = 220 B = [(210)]2 B = k2 B = 1024 kB

1 GB = 230 B = [(210)]3 B = k3 B = 1024 MB

1 TB = 240 B = [(210)]4 B = k4 B = 1024 GB


Example V

Calculate the capacity of computer memory with 16 bit data bus and the 24 bit address bus.

If the address of the row and column of waste by 12 bits, then the complete address is:

.L = 1111 1111 11112  1111 1111 11112 =

.= FFF16  FFF16 = 4'096  4'096 locations =

.= 16'777'216 locations = 16'777'216 / 1024 =

.= 16'384 k = 16 M locations (data)

As the location consists of 16 cells (bits), and it can accept two bytes, the capacity is:

.C = L  2B/location = 32 MB

Thus, one should distinguish between the number of possible stored data from the memory capacity. For each type of microprocessor this relationship differs powerful as Pentium microprocessors can process 64 bit data while the family was the first PC microprocessors that process data of 8 or 16 bits.



3.3.4. Execution of Instructions



Instructions are written in ROM or RAM in your computer as a binary coded records e.g. in ASCII code. Instruction consists of one or more bytes and has two main parts, shown in Figure 3.3.7.

 Composition of instruction
Figure 3.3.7 Composition of microprocessor instructions.

Simpler operations are carried out immediately after the command. Command is mandatory part of instruction. Appendix tags may be an operand or data. If the appendix is not an operand or data then it is the address where the operand or data is located.

Operand is the data on which it operates as the default command adds his number, and data is provided as information for any other instruction.

Read command in the computer memory is transferred to the instruction register of the microprocessor, which decodes the operation and sends the appropriate signal to the control unit as an order (Figure 3.3.3). In operation informs the control unit and whether it is made up of instructions from one or more bytes. If after following command operand is entered into the microprocessor in the second step, or if the third operand is preceded by its address. Only when the entire microprocessor receives an instruction control unit sends the appropriate signal to where it should be.

A series of such instructions, deliberately and logically related, makes a program to solve the challenge. Since the commands executed successively, program counter increases its content for one (+1) and also indicates storage of previous content, or commands or operands in microprocessor or memory. Below are a few instructions in assembler (programming language closest to the machine) for the processor MC 68000.

     :               ; usual procedure
 pushall             ; to allocate
 move.l $0004,a6     ; memory
 clr.l d1
 move.l #(End-BootBl+200),d0
 jsr AllocMem(a6)
 move.l d0,a1
     :
Figure 3.3.8 Sequence of assembler instruction.

Each assembler instruction corresponding to one task microprocessor, which is not the case with the more complex programming languages in which a multitude of instructions are executed in order.

Execution of instructions is performed synchronously at a speed determined by the microprocessor clock. Time needed to move from the memory operand in the register is called INSTRUCTIONAL or MEMORY CYCLE.

Basic steps memory cycles are:

            1.) Bringing the Command-instruction from memory
                instruction register, directly or through the address.
            2.) Decoding command - instructions.
            3.) Execute commands - the instructions.

So, the computer memory is read numeric data representing coded instructions or commands on the basis of which a microprocessor executes operations acting on operand (the one that suffers the operation), and the result of the operand is the change of its contents, activate certain logic circuits or to generate a control signal a part of both.

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