4.8.3 MARIE's Instruction Set
MARIE has a very simple instruction set (yet it is reasonably powerful).
Instructions need to be coded into binary. Each word in our computer is 16 bits wide, and (it
appears that) we need 12 bits for at least one operand, so that means that we have 4 bits
for the instruction code. This means that we can have up to 24 (16) instructions.
Each instruction, therefore, will be represented as a 4-bit number in the range 0-15.
These are the MARIE instructions:
We are cheating on the input and output. In a real computer, numbers have to be converted
from sequences of ASCII characters into their equivalent binary value. We are assuming
that all input values have automatically been converted when they get into the Input
register. And we will assume that the reverse process also happens automatically for the
Output register.
Note that all instructions must be in binary form for the computer to be able to execute
them (which makes sense since we can only put zeros and ones into the computer's
memory).
However, there are two other ways to represent MARIE's instructions:
1. In Hex (column 2 of table above)
2. With an English word (e.g. Load, Store, etc., in column 3 of the table above)
The instructions in numeric format are called machine language.
The instructions in the form of English words are called assembly language.
4.8.4 Register Transfer Notation
Each instruction can be broken down into smaller steps. These smaller steps are called
microinstructions and they can be represented by using something called register
transfer notation.
A register transfer notation instruction represents the movement of data from one register
to another in the CPU. To be able to do this, the registers must be connected:
Note that for each instruction below, we assume that the following micro-instructions (which
are the same for all instructions) have been executed.
Fetch




the instruction
MAR  PC
MBR  M[MAR]
IR  MBR
PC  PC + 1
NOTE: In the places where I use the notation "IR[Addr]" (see below), the author used "X".
My notation more accurately describes what is going on.
After the instruction has been fetched (above), the instruction is executed:
Load X (op code = 1)
 MAR  IR[Addr]
 MBR  M[MAR]
 AC  MBR
NOTE: In the following instruction, the reason that the first two microinstructions can be
executed in parallel is that they use different busses!
Store X (op code = 2)
 MAR  IR[Addr],
 M[MAR]  MBR
Add X (op code = 3)
 MAR  IR[Addr]
 MBR  M[MAR]
 AC  AC + MBR
Subt X (op code = 4)
 MAR  IR[Addr]
 MBR  M[MAR]
 AC  AC - MBR
Input (op code = 5)
 AC  InREG
Output (op code = 6)
 OutREG  AC
MBR  AC
Halt (op code = 7)
 No register transfer operations are performed. The program stops.
Skipcond
Note that this is actually 3 different instructions. The four bits of the op code are the same,
but the instruction is effectively extended by two more bits. If you were writing instructions
in assembly language, this instruction would most likely have 3 different forms:
Skipcond Positive
 If AC > 0 then PC  PC + 1
Skipcond Negative
 If AC < 0 then PC  PC + 1
Skipcond Zero
 If AC = 0 then PC  PC + 1
Jump X
 PC  IR[Addr]
4.9 Instruction Processing
4.9.3 MARIE'S I/O
Actual input and output on a real computer are somewhat complicated. MARIE makes one
significant change from real CPUs: it glosses over the complications of input and output
(and we should be glad that it does – real I/O is ugly).
INPUT: When an Input instruction is executed on MARIE, the computer simply waits until
the user presses a key (or keys) on the keyboard and presses the Enter key. Whatever the
user types is copied directly into the inReg register, and then immediately (and
automatically in this case) copied into the Accumulator register.
OUTPUT: When an Output instruction is executed on MARIE, the computer copies the value
in the Accumulator register to the outReg register, which causes it to be displayed on the
output device.
4.10 A SIMPLE PROGRAM
A program to add two numbers from memory.
Hex Address
Assembly language
instruction
Machine language
instruction (Hex)
100
Load 104
1104
101
Add 105
3105
102
Store 106
2106
103
Halt
7000
104
0023
0023
105
FFE9
FFE9
106
0000
0000
Things to note:
 Program starts at location 100, not 0
 All addresses are in HEX
 All data are in HEX
 All instructions are in HEX
EVERY Assembly language instruction has a corresponding machine language instruction.
Machine language and assembly language are equivalent. Here is the difference: Assembly
language instructions are meant to be read by a person; machine language instructions are
meant to be read by a computer.
NOTE: The register transfer notation for this program is on page 205 in the textbook.
Do the following:
1. Assemble this program by hand.
2. Execute this program by hand.
3. Use the MARIE editor to enter the assembly language version of the program.
4. Use the MARIE assembler to assemble the program into machine language.
5. Use the MARIE simulator to run the program.