Memory reference instructions used in computer architecture is well demonstrated with examples. It will probably help you understand each referencing instructions.

1. MEMORYREFERENCE
INSTRUCTIONS
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Presented by: Rabin BK
BSc.CSIT 3rd Semester

2. Introduction to Memory Reference Instructions
Some terminologies
Memory Reference Instructions
References
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3. There are seven different memory-reference instructions
Actual execution of the instruction in the bus system requires a sequence of
microoperations as data in memory cannot be processed directly
Microoperations are needed for the data to be read from memory to a
register to operate them on logic circuits
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Introduction to Memory Reference Instructions
Symbol Operation Decoder
AND D0
ADD D1
LDA D2
STA D3
BUN D4
BSA D5
ISZ D6

4. Effective address (EA)
• Any operand to an instruction which references memory
• Basically enclosed inside a square brackets
• Calculated as: EA = Base + (Index*Scale) + Displacement
• Displacement — An 8-, 16-, or 32-bit value.
• Base — The value in a general-purpose register
• Index — The value in a general-purpose register
• Scale factor — A value of 2, 4, or 8 that is multiplied by the index value
DR → Data Register
AR → Address Register
IR → Instruction Register
PC → Program Counter
AC→ Accumulator
SC → Sequence Counter
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Some terminologies

5. AND to AC
 Performs the AND logic operations on pairs of bits in AC and the
memory word specified by the effective address
 Two timing signals are needed
• In T4 transfering operand from memory into DR
• In T5 transfering result of AND logic operation between the contents
of DR and AC
• In T5 SC is cleared to 0 and control is transfered to T0 to start a new
instruction cycle
 Example:
• D0T4: DR←M[AR]
• D0T5: AC←AC∧ DR, SC←0
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Instructions

6. ADD to AC
 Adds the contents of memory word specified by the effective
address to the value of AC
 Sum is transferred into AC and the output carry Cout is transferred to
the E(extended accumulator) flip flop
 Two timing signals are needed but decoder D1 instead of D0
 Example:
• D1T4: DR←M[AR]
• D1T5: AC←AC+DR, E←Cout SC←0
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Instructions cont...

7. LDA:Load to AC
 Tranfers the memory word specified by the effective address to AC
 Necessary to read the memory word into DR first and transfer the
contents of DR into AC
 there is no direct path from bus into AC
 to maintain one clock cycle as well
 Example:
 D2T4: DR←M[AR]
 D2T5: AC←DR SC←0
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Instructions cont...

8. STA:Store AC
Stores the content of AC into the memory word specified by the
effective address
 The output of AC is applied to the bus and the data input of
memory is connected to the bus
 Example:
 D3T4: M[AR]←AC, SC←0
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Instructions cont...

9. BUN:Branch Unconditionally
 PC is incremented at time T1 to prepare it for the address of the next
instruction in the program sequence
 BUN transfers the program to the instruction specified by the
effective address
 Allows the programmer to specify an instruction out of sequence
and we say that the program branches (jumps) unconditionally
 Example:
 D4T4: PC←AR SC←0 (resetting SC transfers control to T4)
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Instructions cont...

10. BSA:Branch and Save Return Address
 Useful for branching to a portion of the program called a subroutine
or procedure
 When executed, it stores the address of the next instruction in
sequence (which is available in PC) into a memory location
specified by the effective address
 (Effective address + 1) is then transferred to PC to serve as the
address of the first instruction in the subroutine
 The return to the original program is accomplished by the BUN
instruction placed at the end of the subroutine
 Example:
 D5T4: M[AR]←PC, AR←AR+1
 D5T5: PC ← AR, SC←0
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Instructions cont...

11. ISZ:Increment and Skip if Zero
 Increments the word specified by the effective address
 If the incremented value is equal to 0, PC is incremented by 1
 When a negative number(in 2's compelement) stored in memory word is
repeatedy incremented by 1 it eventually reaches zero
 At this time PC is incremented by one in order to skip the next
instruction in the program
 It is necessary to read the word into DR, increment DR and store the
word back into memory since it is not possible to increment a word
inside the memory
 Example:
 D6T4: DR←M[AR]
 D6T5: DR←DR+1
 D6T6: M[AR] ← DR, if (DR=0) then (PC←PC+1), SC←0
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Instructions cont...

12. References
• Dasgupta, S., Computer Architecture: A Modern Synthersis, Vol. 2 New
York: John Wiley, 1989
• M.Morris Mano, Computer System Architecture, Pearson, Third Edition
• https://www.tortall.net/projects/yasm/manual/html/nasm-effaddr.html
• http://faculty.cs.niu.edu/~berezin/463/notes/addrmode.html
• https://everything2.com/title/Effective+address
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13. Queries
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