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# Memory reference instruction

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Sanjeev Patel 4x

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### Memory reference instruction

1. 1. MEMORY REFERNCE INSTRUCTIONS PRESENTED BY: SURBHI
2. 2. MEMORY REFERENCE INSTRUCTIONS <ul><li>The bits in IR 12 to 14 determines the memory reference instruction type if the bits were from 000 to 110. </li></ul><ul><li>Used here a 3 by 8 decoder is used to decode the 3 bits to 8 lines D0 to D7, although D7 not used here. </li></ul><ul><li>The effective address of the instruction is in AR and was placed there during timing signal T2 when I = 0, or during timing signal T3when I = 1. </li></ul><ul><li>Data from memory must be accessed to processor during T4. </li></ul>
3. 3. AND TO AC D 0 T 4 : DR  M[AR] D 0 T 5 : AC  AC DR, SC  0 ADD TO AC D 1 T 4 : DR  M[AR] D 1 T 5 : AC  AC+ DR, E  COUT, SC  0
4. 4. LDA : LOAD TO AC D 2 T 4 : DR  M[AR] D 2 T 5 : AC  DR, SC  0 STA : STORE AC D 3 T 4 : M[AR]  AC, SC  0
5. 5. BUN : BRANCH UNCONDITIONALLY D 4 T 4 : PC  AR, SC  0 BSA: BRANCH AND SAVE RETURN ADDRESS D 5 T 4 : M[AR]  PC, AR  AR+ 1 D 5 T 5 : PC  AR, SC  0
6. 6. 20 PC=21 AR=135 136 20 21 135 PC=136 (b) memory and pc after execution (a) Memory ,PC AND AR AT TIME T4 MEMORY MEMORY Example of BSA 0 BSA 135 Next instruction Subroutine 1 BUN 135 0 BSA 135 Next instruction Subroutine 1 BUN 135
7. 7. ISZ : INCREMENT AND SKIP IF ZERO <ul><li>D 6 T 4 : DR  M[AR] </li></ul><ul><li>D 6 T 5 : DR  DR+1 </li></ul><ul><li>D 6 T 6 : M[AR]  DR, if(DR=0) then (PC  PC + 1, SC  0 </li></ul>