11.5.1.3 analyze a simple program written in the language of assembler
Programming in Little Man Computer (LMC)
LMC online is a simulator for getting to know the low-level programming language Assembler.
1 - space for a typing program
2 - button for compiling the program
3 - button RUN the whole program, STEP - line by line
4 - registers of CPU
5 - the memory location
6 - input data
7 - output data
LMC contains a set of commands called mnemonics.
Mnemonic Code
Numeric Code
XX is the cell number in the memory compartment.
Instruction
INP
901
Input data
OUT
902
Output data
ADD
1XX
Add data
SUB
2XX
Subtract data
STA
3XX
Store data
LDA
5XX
Load data
BRA
6XX
Branch to a specified cell
BRZ
7XX
If 0, branch to a specified cell
BRP
8XX
If 0 or positive, branch to a specified cell
HLT
000
Break execution
DAT
Treat content as data
Simple program:
Task #1
Write a program that asks for a number and prints it out.
00
INP
input number to accumulator
01
STA 06
store number from the accumulator to a memory location (address) 10
02
OUT
output number from the accumulator
03
HLT
stop program
Memory location after the execution of the program:
Using memory location define what will be displayed
Answer: 23
Format of a machine code instruction
Opcode (Operation Code):
Specifies the operation to be performed.
It's a unique set of bits that corresponds to each operation, such as ADD, SUB, LDA, STA, etc.
Operand(s):
These specify the data or the location of data on which the operation will act.
Operands can be:
Immediate values: Direct values to be used in the operation. For example, in the instruction ADD 5, the number 5 is an immediate operand.
Task #2
Write a program that stores a pair of numbers in variables called FIRST and SECOND. The two numbers should be added together. The answer should be stored in a variable called ANSWER and then output.
00
INP
input 1st number to accumulator
01
STA FIRST
store number from the accumulator to a memory location (address) FIRST (08)
02
INP
input 2nd number to accumulator
03
STA SECOND
store number from the accumulator to a memory location (address) SECOND (09)
04
ADD FIRST
add a number from memory location FIRST (9) to number in the accumulator
05
STA ANSWER
store number from the accumulator to a memory location (address) ANSWER (10)
06
OUT
output number from the accumulator
07
HLT
stop program
08
FIRST DAT
09
SECOND DAT
10
ANSWER DAT
Task #3
Write a program that stores two numbers and prints out the largest number.
00
INP
input 1st number to accumulator
01
STA FIRST
store number from the accumulator to a memory location (address) FIRST (12)
02
INP
input 2nd number to accumulator
03
STA SECOND
store number from the accumulator to a memory location (address) SECOND (13)
04
SUB FIRST
subtract a number from memory location FIRST (12) from the number in the accumulator
05
BRP SECBIG
if an accumulator value is more than 0, go to label SECBIG (09)
06
LDA FIRST
load to accumulator a number from memory location FIRST (12)
07
OUT
output number from the accumulator
08
HLT
stop program
09
SECBIG LDA SECOND
load to accumulator a number from memory location SECOND (13) if in (05) incorrect condition
10
OUT
output number from the accumulator
11
HLT
stop program
12
FIRST DAT
13
SECOND DAT
Questions:
1. What is the mnemonic code?
2. Name mnemonics of LMC and explain each.
Exercises:
Ex. 1 Fill the gaps
Ex. 2 Write a LMC program (Author: Litvinova Olga - CS teacher of NIS Pavlodar)
Ex. 3 Define values of the memory location (Author: Litvinova Olga - CS teacher of NIS Pavlodar)
Ex. 4 Fill the trace table
Ex. 5 Fill the trace table
Ex. 6 Write a program in LMC for calculating (A-C)+(B-D).
