2. Tracing a simple MARIE program

Tracing a simple MARIE program

Consider the following program:

Address	Instruction	Contents of Memory
$100$	Load $104$	$0\text{x}1104$
$101$	Add $104$	$0\text{x}3105$
$102$	Store $104$	$0\text{x}2106$
$103$	Halt	$0\text{x}7000$
$104$	$0023$	$0\text{x}0023$
$105$	$\text{FFE}9$	$0\text{xFEE}9$
$106$	$0000$	$0\text{x}0000$

$001.\text{ Load }104$

Step	RTL	PC	IR	MAR	MBR	AC
(initial values)		$100$	-	-	-	-
Fetch	MAR $\leftarrow$ PC	$100$	-	$100$	-	-
	IR $\leftarrow \text{M}[$ MAR $]$	$100$	$1104$	$100$	-	-
	PC $\leftarrow$ PC $+1$	$101$	$1104$	$100$	-	-
Decode	MAR $\leftarrow$ IR $[11-0]$	$101$	$1104$	$104$	-	-
	(Decode IR $[15-12]$)
Get operand	MBR $\leftarrow \text{M}[$ MAR $]$	$101$	$1104$	$104$	$0023$	-
Execute	ACC $\leftarrow$ MBR	$104$	$1104$	$104$	$0023$	$0023$

$002.\text{ Add }105$

Step	RTL	PC	IR	MAR	MBR	AC
(initial values)		$101$	$1104$	$104$	$0023$	$0023$
Fetch	MAR $\leftarrow$ PC	$101$	$1104$	$101$	$0023$	$0023$
	IR $\leftarrow \text{M}[$ MAR $]$	$101$	$3105$	$104$	$0023$	$0023$
	PC $\leftarrow$ PC $+1$	$102$	$3105$	$104$	$0023$	$0023$
Decode	MAR $\leftarrow$ IR $[11-0]$	$102$	$3105$	$105$	$0023$	$0023$
	(Decode IR $[15-12]$)
Get operand	MBR $\leftarrow \text{M}[$ MAR $]$	$102$	$3105$	$105$	$\text{FFE}9$	$0023$
Execute	ACC $\leftarrow$ ACC $+$ MBR	$102$	$3105$	$105$	$\text{FFE}9$	$000\text{C}$