1. Sequencing, Selection, and Loops in Machine Language
Computer Science 101
Sequencing, Selection, and Loops
in Machine Language

2. Sequencing Supported by the linear structure of memory
Begin
<statement 1> …
<statement n>
End
Sequencing
Supported by the linear structure of memory
After the PC is incremented, the instruction fetched is the the next one in a linear sequence
A normal termination occurs when the HALT instruction is fetched and executed

3. If <condition>
Begin
<statement 1> …
<statement n>
End
Selection
Selection (one-way if) statements use two types of instructions
A comparison
A conditional jump

4. Comparisons Require two operands (example, X > Y)
The COMPARE opcode 0111 compares the data in the memory cell at the given address to the contents of the data register R
The result of the comparison is deposited in the condition code register CCR

5. The Condition Code Register
Comparison State of CCR
CON(X) > R
CON(X) = R
CON(X) < R 1 1 1
CON(X) means contents of memory cell at address X

6. Example: Compare X and YIR 1 1 1 1 1 1 Y X R 1 1
CCR 1
X = Y
RAM 1 1

7. Example: Compare X and YIR 1 1 1 1 1 1 Y X R 1 1
CCR 1
X > Y
RAM 1

8. Example: Compare X and YIR 1 1 1 1 1 1 Y X R 1 1
CCR 1
X < Y
RAM 1

9. Steps Required Comparisons require two steps:
Load the second operand into register R
Compare to the first operand (a memory address)
LOAD Y
COMPARE X

10. JUMP Instructions
The JUMP instructions check the condition code and transfer control to the instruction at location X if the code is set in a certain way.
Binary opcode Operation CCR
JUMP X Any value
JUMPGT X 100
JUMPEQ X 010
JUMPLT X 001
JUMPNEQ X Any value but 010

11. Example If Statement High-level pseudocode Low-level pseudocode
// Absolute value
If X < 0
Set X to -X
// Absolute value
1. LOAD ZERO
2. COMPARE X
3. JUMPGT LINE 6
4. SUBTRACT X
5. STORE X
6. Rest of code
These two lines are skipped if X > 0

12. In Machine Code LOAD 000000000000 1 COMPARE 000000000001 1 1 1 1 1 11
COMPARE 1 1 1 1 1 1
JUMPGT 1 1 1 1
SUBTRACT 1 1 1 1 1
STORE 1 1 1 1
OUT 1 1 1 1 1 1
HALT 1 1 1 1 X 1 1 1
ZERO

13. If/Else and Loops
Comparisons and jumps are also used for if/else statements and loops
Typically use two jumps, one conditional and the other unconditional

14. Example: Output Sum of 1 .. 10 Set sum to 0 Set count to 1
While count <= 10 do
Set sum to sum + count
Increment count
Output sum
1 clear sum
2 load one
3 store count
4 load count
5 compare eleven
6 jumpeq line 12
8 add sum
9 store sum
10 increment count
11 jump line 4
12 out sum
13 halt

15. Example: Output Sum of 1 .. 10 1 clear sum 2 load one 3 store count
4 load count
5 compare eleven
6 jumpeq line 12
8 add sum
9 store sum
10 increment count
11 jump line 4
12 out sum
13 halt
(zero)
(one)
(eleven)
(sum)
(count)