Components of the Computer

• CPU -- Central Processing Unit
  The CPU is the brain of the computer and it distinguishes between the different types of computers. It's the fastest component of the computer. Inside the CPU there is an internal bus.
  
• Chip
  The chip of a computer depends on the type of computer.
  
• How a Chip is Produced
  The CPU is made with reverse photography. A picture is taken and the negative is put on hot silicon wafers (coated material etched away when struck by light). Light shines through the negative and circuits are etched into the chips.
  
• Clock Speed
  The speed of the CPU is measured in clock speed, or hertz. Every hertz is equal to one cycle per second. The faster the ticks are the faster the cycle takes. This does not have anything to do with the speed of operations. The first IBM PC ran at 4.77 MHz (megahertz). The next PC was the IBM XT which ran at the same speed but had a 10-Mg hard drive. Afterwards came the IBM AT which ran at 6 MHz. When this came out, people realized that for a quarter more they could buy an 8 MHz clock, so IBM changed the BIOS so the 8 MHz chip wouldn't work. But this change in BIOS only got the computer to check for it at startup and people were able to get around it. A problem with clock speeds is that the faster the clock works, the hotter it gets. Making the components smaller and therefore need less power and electrons can reduce heat. .
  
• Bus Width (Word Size)
  The bus width is the amount of bits that can be transported at a time.
  
• Address Space
  A certain amount of digits are allocated to the address which is known as the address space. The more bits in an address, the more addresses we can have. If there's an 8-bit address, the address space is 2(exp n)-1 which is 2(exp 8)-1 = 255 addresses.
  
  The maximum addressable memory that the CPU can talk to is determined by the number of bits allocated for an address. The CPU has physical connections with RAM to bring in data. Word size refers to the number of data lines a chip has. The first Apple chip (6502) had 8 data lines and 8 address lines (max 256 addressable bytes). To be able to have more addresses, the address is sent over in two clock cycles and the data is sent over in the third cycle. This allowed 64,000 (64K) addressable bytes. The IBM introduced the 8086 which had 16 data lines and the 8088 had 8 data lines. Since IBM was trying to build the processors in one year, they bought the components, which were 8 bit processors. IBM ended up sticking with the 8088 which had an 8 bit external bus and 16-bit internal bus (slightly faster). The 8088 had 20 address lines. This is no good because the computer can only talk in groups of 8 bits. As a result an Addressing Scheme was created. This is segmented architecture - addresses are constructed by giving you a segment and an offset. In this situation, there was a 16 bit segment, and a 16 bit offset. And as a result there was 220 addressable memory, or 1M. This is an efficient use of memory and the bus. Models of memory: Tiny model- all the program and data fit within 64K and therefore all addresses would be 16 bits at once (2 cycles per fetch - quicker) Large model- The programs fit into 4K and the data is in another 64K. A memory location can be referred to in different ways and can cause a protection exception. This is when two programs try to access the same location and crashed the computer. It adds additional complexity. Some solutions Microsoft wrote DOS for IBM. The upper memory (1M-s40) was reserved for the ROM and various adapter boards (they didn't think that they would ever need that much memory). The business world only used that PC for word processing, and needed very sharp text. IBM allocated memory for a monochrome monitor. Kids used the PC for games and didn't need high resolution. So, they used TV screens that allowed only 40 characters to go across. And IBM allocated memory for a graphics adapter. IBM also allocated 0 t0 640K of memory for writing DOS.