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21--MARIE--06 (1)

# 21--MARIE--06 (1) - How instructions actually get executed...

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How instructions actually get executed Machine instructions are executed by executing a sequence of register transfer instructions  – sometimes  called  micro-instructions If you look at the microinstructions for the MARIE instruction set, you will see that the longest is the JnS  instruction – 7 micro-instructions. We need a way to make sure that each of these micro-instructions  happens in order.  Imagine a 3-bit counter: Label the outputs T 0  through T 7 . If you want three things to happen in sequence, use T 0  as an enable for  the first event, T 1  as an enable for the second event, etc. Consider the RTN micro-instructions. Each specifies a register on the left side and one on the right side.  Example: The following are the micro-instructions for the LOAD instruction MAR IR[addr] MBR M[MAR] AC MBR

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If you look at the picture on page 192 of Null, you will see that each register has been assigned a number  from 0 through 7, which means that each register can be specified with a 3-bit number: So the numbers that correspond to the three instructions above are: MAR IR[addr] MAR is 1, IR is7 MBR M[MAR] MBR is 3, M is 0 AC MBR AC is 4, MBR is 3 Each of these can be represented by a 3-bit number. So to specify a single RTN instruction, we need 6  bits, 3 for the destination, and 3 for the source. The "code" for each of the micro-instructions given above  is: MAR IR[addr] 001 111 MBR M[MAR]
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21--MARIE--06 (1) - How instructions actually get executed...

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