Building the ALU for the 8 bit computer.

• We need to figure out how the ALU works to be able to figure out "how" the jumpC instruction works.
  • Actually this is not that hard, it is just the carry out of the last bit of the "adder", if we only knew what the adder was.
• We could look at Ben's schematic about 1/8 of the way down the page.
  • He seems to have the a register hooked into to 74283 chips.
  • And the b register is hooked in through a series of xor chips.
  • The carry bit is set from the 4th carry out of the top chip.
  • 
• Let's look at the 74283.
  • The datasheet
  • This is a 4 bit adder.
  • There are 4 A inputs (A1- A4)
  • 4 b inputs (B1-B4)
  • A carry in (C0)
  • 4 sum outputs (Σ1 - Σ4)
  • A carry out (c4)
  • A positive voltage
  • A ground
• We can hook up a version in digital
  • 
  • Digital File
• Half of Eater's ALU
  • 
  • digital file
• Could we build the adder ourselves?
  • We start with a 1 bit adder.

    a	b	cin	cout	sum
    0	0	0	0	0
    0	0	1	0	1
    0	1	0	0	1
    0	1	1	1	0
    1	0	0	0	1
    1	0	1	1	0
    1	1	0	1	0
    1	1	1	1	1

  • optimize f=Σm(3,5,6,7)
  • optimize f=Σm(1,2,4,7)
  • Turn this into a circuit.
• Build a 4 bit ripple carry adder.
• A Multiplexer
  • Discussed in 4.3 of Carpanelli
  • Has one output line
  • Has n control lines
  • Has 2ⁿ input lines, numbered 0 to 2ⁿ-1
  • The n control lines select between the input lines
  • Sometimes there is an enable port
  • A component in Digital
  • Multiple but 74153
  • 74153 datasheet.
  • We can build one with the ideas that a·1 = a, a+0 = a
• Note that to subtract we flip the bits (inverters) and add 1
  • We can handle the add one with a carry in of 1.
• Thus the 8-bit ALU
  • Input
    • A - an 8 bit data source
    • B - an 8 bit data source
    • Subtract - a 1 bit, 0 indicates add, 1 indicates subtract
  • Output
    • Sum the sum
    • C_out the carry out of the last bit.
  • Build this.
• But that still doesn't answer the question, when is the carry out bit set?
  • For add
  • For subtract