3-4. MARIE

MARIE

Machine Architecture that is Really Intuitive and Easy

Specification

• Binary, two’s complement data representation
• Stored program, fixed word length data and instructions
• $4\text{K}$ words of word-addressable main memory
• $16\text{-bit}$ word size
• $16\text{-bit}$ instructions ($4$ for op code, $12$ for address)
• $16\text{-bit}$ ALU
• Seven registers for control and data movement

Architecture

Below is an overview of the MARIE architecture.

The ratio of address, control and data lines varies between computer systems.

• More devices on the bus mean we want more control lines.
• More addresses in memory mean we want more address lines.
• Larger word size, means more data lines.

Bus

All of MARIE’s registers are connected with each other and with main memory through a common data bus.

• Each device on the bus is identified by a unique number. This number is set on the control lines whenever the corresponding device is required to carry out an operation.
• Separate connections are also provided between the following registers:
  • ACC and MBR
  • ALU with the ACC and MBR

MARIE ISA

Instruction Set Architecture

A computer’s instruction set architecture (ISA) specifies the format of its instructions and the primitive operations that the machine can perform.

• The ISA is an interface between a computer’s hardware and its software.
• Some ISAs include hundreds of different instructions for processing data and controlling program execution.

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MARIE ISA

MARIE’s ISA contains thirteen instructions.

Hex	Binary	Instruction	Meaning
$0$	$0000$	JnS X	Jump and Store: Stores PC at address $X$ and jumps to $X+1$.
$1$	$0001$	Load X	Load contents of address X into ACC
$2$	$0010$	Store X	Store the contents of the ACC at address X
$3$	$0011$	Add X	Add the contents of address X to the ACC
$4$	$0100$	Subt X	Subtract the contents of address X from the ACC
$5$	$0101$	Input	Input a value from input device into the ACC
$6$	$0110$	Output	Output the value in the ACC to output device
$7$	$0111$	Halt	Terminate the program
$8$	$1000$	Skipcond C	Skip the next instruction on condition (C), if $C$ is: - $000$: Skips if $AC<0$ - $400$: Skips if $AC=0$ - $800$: Skips if $AC>0$
$9$	$1001$	Jump X	Load the value of X into PC
$A$	$1010$	Clear	Set the value of ACC to $0$.
$B$	$1011$	AddI X	Use the value at $X$ as the actual address of the data operand to add to ACC
$C$	$1100$	JumpI X	Uses the value at $X$ as the address to jump to
$D$	$1101$	LoadI X	Loads value from indirect address to ACC.
$E$	$1110$	StoreI X	Stores value in ACC at the indirect address.

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Register Transfer Language

Register Transfer Language

Each of MARIE’s instructions consist of a sequence of micro-operations: data movements between registers as well as arithmetic & logical operations.

The exact sequence of micro-operations that are carried out by an instruction can be specified using the register transfer language (RTL).

We use the notation $M[X]$ to indicate the actual data value stored in a memory location $X$, and $\leftarrow$ to indicate the transfer of bytes to a register or memory location.

Example RTL for the LOAD X instruction:

$$\begin{array}{rl}MAR& \leftarrow X\\ MBR& \leftarrow M[MAR]\\ AC& \leftarrow MBR\end{array}$$

Example RTL for the ADD X instruction:

$$\begin{array}{rl}MAR& \leftarrow X\\ MBR& \leftarrow M[MAR]\\ AC& \leftarrow AC+MBR\end{array}$$

Example RTL for the JUMP X instruction:

$PC\leftarrow X$

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