Microprocessors
Contents |
CPU
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Central Processing Unit is the brain of the computer. In reality the CPU is nothing more than a calculator. The only difference is the speed. Let's see how this box works. First thing we need to figure out is how to talk to the box. If there is a little man sitting inside the box, how do we let that man know that we need some calculations done? This is where external data bus come into the picture. |
Pin Number Description 1 GND - Ground 2-8 ADDR - Address Bus 9-16 DATA - Address Data Bus 17 NMI - Non-Maskable Interrupt 18 INTR - Interrupt Request 19 CLK - Clock 20 GND - Ground 21 RESET - Reset 22 READY - Ready 23 TEST - Test (Active Low) 24 INTA - Interrupt Acknowledge (Active Low) 25 ALE - Address Latch Enable 26 DEN - Data Enable (Active Low) 27 DT/R - Data Transmit / Receive 28 IO/M - Status Line 29 WR - Write (Active Low) 30 HLDA - Hold A 31 HOLD - Hold 30 HLDA - Hold A 32 RD - Read (Active Low) 33 MN/MX - Minimum or Maximum Mode 34 SSO - Status Line (Active Low) 35-38 ADDRESS/STATUS - Address/Status Information 40 Vcc - Positive Power Supply |
External Data Bus
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Inside the CPU There are registers. Registers are tables to hold information that needs to be worked upon. Some registers are AX, BX, CX and DX. The external data bus is where the CPU receives information that the CPU needs to add, subtract, multiply or divide and then send it back on the external data bus. This data bus is blue is the picture on the right. |
Clock
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Clock is similar to hitting the operation key on your calculator. For example if you want to add 2 + 3, then hitting the + sign will be an indication of a Clock cycle to go to the next instruction. On the CPU, there is one wire that is called the CLK wire which is the clock wire. When this CLK wire is charged it tells the CPU to go to the next instruction. Each time the CLK wire is charged it is called a clock cycle. The maximum number of clock cycles your CPU can handle is called the clock speed.
The system crystal is the metronome for the CPU. The registers in the CPU are microscopic semi conductor circuits called TTL logic. Now once you have done your computation you will need to store your results. Or the CPU needs to read the instruction from somewhere fast enough to process them quickly. This is where memory comes into please. |
Memory
| Memory is where all the instructions that the CPU needs to process are located. There are two types of memory. RAM and ROM. Memory is no more than small capacitors that hold a charge. |
Address Bus
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Address bus is the connection between the CPU and the RAM. The Chip in controls the physical memory is called the Memory Controller Chip (MCC). Let's take an example of 20 wires running from the CPU to the MCC. By turning these 20 wires on and off the CPU can address 220 1,048,576 different ways of telling MCC which piece of information the CPU needs from memory. Since each pattern points to one line of code and since each line of RAM is one byte, if you know the number of wires in the address bus, you know the maximum amount of RAM that a particular CPU can handle. Since the 8088 has 20-wire address bus, the most RAM it can handle is 220 or 1,048,576 bytes. The 8088, therefor has an address space of 1,048,576 bytes.
Next question is, which pattern goes to which line of RAM. When the CPU turns off all the address bus wires, it wants the first line of RAM; when it turns on all of the wires, it wants the 1,048,576th line of RAM. Some external Data Bus wires and address bus wires overlap, this normal and is called multiplexing. |
Memory Access
| Processors | Address Bus Width (bits) | Address Bus Width (bits) |
|---|---|---|
| 8088, 8086 | 20 | 1 MB |
| 80286, 80386SX | 24 | 16 MB |
| 80386DX, 80486DX, 80486SX, 80486DX2, 80486DX4, AMD 5x86, Cyrix 5x86, Pentium, Pentium OverDrive, Pentium with MMX, Pentium with MMX OverDrive, 6x86, K5, K6, 6x86MX | 32 | 4 GB |
| Pentium Pro, Pentium II | 36 | 68 GB |
You can address 68 gig of memory if your MCC allows you. The limitation here will be your Memory Controller Chip and the actual memory slots on your motherboard.
