Registers could potentially be the most important part of a computer. A register temporarily stores a value during the operation of a computer. The 8-bit computer described in this Instructable has two registers attached to its ALU, a register to store the current instruction and a register for the output of the computer.
Depending on the chip, a register will have 2 or 3 control pins. The registers that we will be using have two control pins: output enable and input enable (both active when low). When the output enable pin is connected to ground the currently stored binary word is sent out across the output pins. When the input pin is connected to ground the binary word present on the input pins is loaded into the register.
An example of the use of a register on a computer is the accumulator on the ALU (arithmetic logic unit that performs mathematical operations). The accumulator is like the scratchpad for the computer that stores the output of the ALU. The accumulator is also the first input for the ALU. The B register is the second input. For an addition operation, the first value is loaded into the accumulator. After that the second value to be added to the first value is loaded into the B register. The outputs of the accumulator and B register are fused open and are constantly feeding into the ALU. The final step for addition is to transfer the output of the operation into the Accumulator.
Registers all operate on a shared data line called the bus. The bus is a group of wires equal in number to the architecture of any CPU. This is really putting the horse before the cart considering bus width is the defining measurement for CPU architecture. Since a digital 1 means positive voltage, and a 0 means grounding, it would be impossible to have all registers share the same bus without giving them the ability to selectively connect and disconnect from the bus. Luckily for us, there is a third state between 1 and 0 that is ambivalent to current imput that works great for this. Enter the tri-state buffer: a chip that allows you to selectively connect groups of wires to a bus. Using some of these tri-state-buffers, you can have every register and chip on the entire computer needing of communication share the same wires as a bus. In the case of my computer, it was an 8-wire wide band of breadboard slots that spanned the bottom pins of the breadboard. Experiment around with busses, since they carry all of the information from piece to piece in the computer a faulty buss could mean erroneous data that ripples down the line.
The great thing about building an 8-bit computer is that most parts will cost you less than a dollar a piece if you buy them from the correct place. I purchased 90% of my parts from Jameco Electronics and I have been completely satisfied with their services. The only parts I have really bought from anywhere else are the breadboards and breadboard wires (and the Numitron tubes). These can be found considerably cheaper on sites like Amazon. Always be sure to make sure the parts that you are ordering are the correct ones. Every part that you buy should have a datasheet available online that explains all of the functions and limitations of the item that you are buying. Make sure to keep these organized as you will be using many datasheets in the construction of your computer. To help you with your computer I will list the parts that I used for mine:
74161 - http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=74161&langId=-1&storeId=10001&productId=49664&search_type=jamecoall&catalogId=10001&ddkey=http:StoreCatalogDrillDownView
4-Bit Register (I use two for each 8-bit register):
74LS173 - http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=74LS173&langId=-1&storeId=10001&productId=46922&search_type=jamecoall&catalogId=10001&ddkey=http:StoreCatalogDrillDownView
74LS157 - http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_46771_-1
16x8 RAM (output needs to be inverted):
74189 - http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=74189&langId=-1&storeId=10001&productId=49883&search_type=jamecoall&catalogId=10001&ddkey=http:StoreCatalogDrillDownView
74LS283 - http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=74LS283&langId=-1&storeId=10001&productId=47423&search_type=all&catalogId=10001&ddkey=http:StoreCatalogDrillDownView
74S244 - http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_910750_-1
74LS86 - http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_295751_-1
74LS08 - http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_295401_-1
74LS02 - http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_283741_-1
74LS04 - http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_283792_-1
CD4029 - http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=4029&langId=-1&storeId=10001&productId=12925&search_type=jamecoall&catalogId=10001&ddkey=http:StoreCatalogDrillDownView
74LS10 - http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_295427_-1
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