{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

21--MARIE--06 (1)

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

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
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
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
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]
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

Page1 / 6

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

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon bookmark
Ask a homework question - tutors are online