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Assignment 2 – Assembly Language solution

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COMPSCI 210
Computer Systems 2
Assignment 2 – Assembly Language
Worth 5%

Introduction
This assignment requires you to write a number of LC-3 assembly language routines leading to
being able to transform a string of digits (and + and – signs) into both the corresponding integer
and the 16-bit floating point value (as in Assignment 1).
You submit this assignment using Canvas. Package all of your programs together into a zip file
and submit the one file. If you submit more than once Canvas adds extra numbers to the
submission. Don’t worry, the markers will always use the most recent one.

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COMPSCI 210
Computer Systems 2
Assignment 2 – Assembly Language
Worth 5%

Introduction
This assignment requires you to write a number of LC-3 assembly language routines leading to
being able to transform a string of digits (and + and – signs) into both the corresponding integer
and the 16-bit floating point value (as in Assignment 1).
You submit this assignment using Canvas. Package all of your programs together into a zip file
and submit the one file. If you submit more than once Canvas adds extra numbers to the
submission. Don’t worry, the markers will always use the most recent one.
Part 1 (1 mark)
This program finds the location (i.e. the bit number) corresponding to the location of the first “1” in
a number.
Call the program part1.asm.
Because this program doesn’t take data from the keyboard you must have a label number in the
code as here:
number .fill b0100000000000001
This number must be loaded into a register in the first line of your program e.g.
.orig x3000
ld r0, number
The program then determines the location of the first “1” in that number and prints the result to the
display. The result from the value above would be:
The first significant bit is 14
If you then modify the value of number to b0000000000000001 the result would be:
The first significant bit is 0
The markers will never enter a value of zero into number.
Part 2 (3 marks)
This program accepts an integer typed in by the user, verifies that the number is valid, and if it is
valid prints the binary version of the number to the display as on the following page.
Call the program part2.asm.
As you can see the program rejects any input which doesn’t start with a + or – sign and prints “The
input is invalid.” to the display.
Also any integer values less than -511 or greater than +511 are rejected. Values like +0123 are
acceptable.
When a value is rejected the program loops back and asks for a new number.
The program stops after printing out the binary representation of the entered number, with a space
between each pair of bits.
page 1
COMPSCI 210 Assignment 2
Part 3 (1 mark)
This program shifts a number 11 places to the left, then ANDs the answer with
b0111100000000000. This means the answer can only have 1s in bits 14 to 11. The answer is then
written to the display as in Part 2.
Call the program part3.asm.
As in Part 1 the number to use must be stored at a label called number and the program begins
with:
.orig x3000
ld r0, number
If the number is b0000000000011011 the output should be:
0 1 0 1 1 0 0 0 0 0 0 0 0 0 0 0
Part 4 (4 marks)
This program accepts an integer between -511 to +511 typed in by user. It works as in Part 2 until
a valid number is entered, then the program prints the binary version of the number as an integer
and follows it by printing the binary representation of the number as a 16-bit floating point
number in the same format as in Assignment 1. Unlike in Assignment 1 the input numbers are
only integers.
Bit 15 is the sign bit.
Bits 14 – 11 is the biased exponent (bias of 7).
Bits 10 – 0 is the mantissa (fractional part).
Remember there is an implicit 1 for the fractional part.
On the next page are some example interactions with the program.
page 2
COMPSCI 210 Assignment 2
You must use subroutines for this program. To assist you to design your code there is a file called
part4.asm which you must add to. You must not add any lines of code before the halt
instruction.
Example input and output.
Zero is a special case.
When writing assembly language the temptation is to treat most values as globally accessible. The
solution to this is to pass parameters and return values on the stack as in higher level languages.
For this assignment you may use globally accessible values when that makes sense e.g. the input
page 3
COMPSCI 210 Assignment 2
buffer used when the user types a number in can be accessed without having to pass its address to
the subroutines which use it.
You should save all registers (except R0) when you enter a subroutine and restore them on exit.
This is not strictly necessary in this assignment because the top-level program doesn’t depend on
registers maintaining their values. On a related point most programming languages keep all of the
code separated from the data. In assembly language that is also a good idea but for this assignment
it may be easier if you keep data associated with a subroutine near that subroutine. It makes it
easier for you to read and fix your code. Also many of the LC-3 addressing modes limit offsets
from the current value of PC to 256 words.
Style (1 mark)
The markers will be looking for nicely formatted code with good comments. Remember to record
what you are using the registers for.
And
Include a comment with your login name (UPI) at the top of each program you submit. Markers
will remove a mark if you have not done this on every program.