Concepts of an address and data, Computer Engineering

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The heart of a computer is often one integrated circuit; one typical circuit is a 68Hc11 Microprocessor. Let us consider a typical 68Hc11 hardware design.

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          The microprocessor has three parallel connection of wires (buses).Attached to these buses are memory storage devices (RAM, EPROM) and input/output connections (serial /parallel).The microprocessor can communicate to any of these devices by means of the buses . The device can be selected by sending down a unique address number on the address bus and the information to be send or received is transferred via the data bus. It is often liken to that of a telephone call, the phone number is the address of the device and the conversation is the data .Within a system ,  it is common to have a description of the allocation of these address' in the form of a memory map.

This is simply a plot of address against data and indicates what address' are used be what devices. A typical memory map for a system is shown. Here the system uses two I.C's namely EPROM (non volatile storage), Ram (volatile storage), with internal parallel and serial port. Let us look at the operation of the microprocessor in a little more detail. The program is assembled from Assembly language into binary and stored in EPROMS, as shown below:

Assembly language program
name ex1  ;Immediate data example
p6811    ;set processor type
org $500  ;Start on external ram
ldaa #$fe  ;load Acc A with fe hex
adda #$1  ;ADD 1 TO a
Address  Assembly Code
000500    86
000501    FE
000502    8B
000503    01
00FFFE    05
00FFFF    00

 On power on the microprocessor issues a reset which causes the processor to go into a vector mode, this in turn reads in the next two bytes starting at address FFFE. The data stored at this location represents the initial program counter (PC) the data is then loaded PC respectively i.e. PC = 0x0500. This causes the processor to read the data at the address 0x500. The microprocessor reads in 0x86 and understands that the first piece of data must be a program command. It interprets 0x86 as a LDAA # data instruction and the data must follow .Therefore the microprocessor increments the PC to 0x501 and reads in the immediate data 0xfe. This is known as the 'FETCH' cycle because the microprocessor has fetched the instruction. The next stage is the 'EXECUTION 'cycle. Here the microprocessor transfers the data 0xfe into accumulator A and increments the PC to 0x502 and fetches the next instruction. A day in the life of the microprocessor is simply one of fetching and executing. Within C we often want to access the memory map directly, in order to talk to I/O devices or alter data in RAM . For this purpose C allows us to use the concept of address pointers.

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