Memory and PC for the 8 bit computer

Objectives

Notes

• Memory
  • Documentation
  • The 8 bit computer has a word size of 8 bits.
  • Therefore memory must be 8 bits wide
  • It has a 4 bit address, so 16 words tall
  • A total of 16x8 = 128 bits.
  • Ben uses the 74x189 chip
    • This is a 64 bit ram
    • 4 Output lines (Q 0-3) for a 4 bit word.
    • 3 address lines (A 1-3) for 16 words.
    • 4 data lines (D 0-3)
    • cs, chip select
    • we, write enable
  • This memory is strange in that the output is inverted.
    • So he uses two 74x04 , 6 input inverters, to fix this.
  • The 74x1157 is a 4 bit wide 2 to 1 MUX
    • A (1-4) and B (1-4) are inputs
    • Y (1-4) is selected output
    • S is select between A and B
    • G is an enable (on low) line
  • His address input is either a 4 bit register, loaded from the bus (74x173) or a 4 position dip switch.
    • He uses a mux to select between them.
  • 
  • His data input is also either the 8 bits from the bus or an 8 position dip switch.
    • He uses two muxes to select between them.
  • 
  • I took a picture before he added a save button to the left of the data entry dip switches
  • I built a digital circuit for this, but it was a mess too!
  • You can see this in action at about 3:00 in This video
  • Yes, he enters his program by hand, enter address, enter code, press save.
• The PC is here
  • And is mostly what you would expect.
  • A tri-state device to control access to the bus
  • A 74x161 binary counter.
    • Four data in lines
    • Five data out lines (carry out of bit 4)
    • A clock
    • A clear
    • A Load
      • If load is low, it loads data from data in on clock pulse
      • Otherwise it increments on clock pulse
    • An enable line.
    • 
    • Implementing this is not too bad.
      • Count to 16 in four bit binary.
      • Note that the first bit flips every other time
      • What will this circuit do?
      • That leads to this circuit
      • 
  • The digital file for counters.
  • 
    • Only 4 bits are sent to the bus
    • The top four are 0.
    • Note this will also receive data from the bus for jump commands
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