Table of Contents
1. Introduction
2. RTL of MARIE Code
2.1 Direct Addressing
2.2 Indirect Addressing

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Introduction

Register Transfer Language or RTL shows how the CPU (Assembler) works. Within the CPU there are many components including:

• AC or Accumulator : intermediate data is stored within the AC
• PC or Program Counter : as the name suggests it counts the current position of the code, each line has it's own address
• MAR or Memory Access Register , stores or fetches the 'data' at the given address
• MBR or Memory Buffer Register , stores the data when being transferred
• IR or Instruction Register

Note that the end of each code, you will need to increment the PC by 1, so:

PC ← PC + 1

RTL of MARIE Code

Direct Addressing

Load X

Load X loads the value from address X into the AC

MAR ← X       # load X (address) into MAR
MBR ← M[MAR]  # load value stored at address into MBR
AC  ← MBR     # load value in MBR into AC

Store X

Store X stores the current value from the AC into address X

MAR    ← X    # load address into MAR
MBR    ← AC   # load AC value into MBR
M[MAR] ← MBR  # write MBR value into the Memory of the address indicated by the MAR

Add X

Add X adds the value stored at address X into AC

MAR ← X           # load X into MAR
MBR ← M[MAR]      # load value stored at address X into MBR
AC  ← AC + MBR    # add value in AC with MBR value and store it back into AC

Subt X

Subt X subtracts the value in AC with the value stored at address X

MAR ← X
MBR ← M[MAR]
AC  ← AC - MBR

Jump X

Jump X jumps to address X

PC ← X

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Indirect Addressing

LoadI X

LoadI X loads the value which is stored at address of the address X into the AC

MAR ← X       # load value X into MAR
MBR ← M[MAR]  # load value stored at address X into MBR
MAR ← MBR     # load value back into MAR (MAR cant write itself)
MBR ← M[MAR]  # load value stored at the address indicate by the value of address X
AC  ← MBR     # Load value into AC.

AddI X

AddI X adds the value which is stored at address of the address X to the value in AC and stores it back into AC

MAR ← X              # load value X into MAR
MBR ← M[MAR]         # load value stored at address X into MBR
MAR ← MBR            # load value back into MAR (MAR cant write itself)
MBR ← M[MAR]         # load value stored at the address indicate by the value of address X
AC  ← MBR + AC       # Load value into AC

JnS X

JnS X or Jumps and Stores: Stores PC at address X and jumps to X+1

MAR    ← X           # loads value X into MAR
MBR    ← PC + 1      # loads value of PC into MBR
M[MAR] ← MBR         # stores value in MBR into address of MAR
AC     ← X + 1       # increments X by 1 and stores it into AC
PC     ← AC          # jumps program counter to the address indicated by the AC

JumpI X

JumpI X uses the value at X as the address to jump to

MAR ← X             # loads value X into MAR
MBR ← M[MAR]        # loads value stored at address X into MBR
PC  ← MBR           # loads the value into PC