Week 07 Laboratory Exercises
Objectives
 to practice using bitwise memory
 to practice manipulating dynamic memory
 to explore arbitrary precision integer arithmetic
Preparation
Before the lab you should reread the relevant lecture slides and their accompanying examples.
Getting Started
lab07
and changing to this directory.
mkdir lab07 cd lab07
There are some provided files for this lab which you can fetch with this command:
1521 fetch lab07
If you're not working at CSE, you can download the provided files as a zip file or a tar file.
Exercise — individual:
Create An addi Instruction
However, each CPU instruction is represented in memory as some particular bit pattern. The CPU is able to decode the instruction in order to determine what operation to perform, and which operands (whether it be a register or immediate value) are involved.
In this exercise, we will construct the bit pattern for an addi
instruction, given operands.
The MIPS documentation guide provides the general bit pattern for an addi instruction, including the opcode and the bits used for each operand.
Instruction  Description  Encoding  

✓  ADDI
 R_{t}, R_{s}, Imm_{16}

R_{t} = R_{s} + Imm_{16}
 001000ssssstttttIIIIIIIIIIIIIIII

The bit pattern here can be broken down into 4 components:

The opcode: in this case, the opcode
001000
is used to distinguish the instruction as anaddi
instruction. 
The two registers:
sssss
andttttt
are used to represent the registers being used as operands for the instruction. Each register number is encoded as a 5bit value  recall that there are 32 generalpurpose registers available in MIPS, numbered$0
to$31
. This means that 5 bits is sufficient to refer to a register.  An immediate value: The last 16 bits are used to represent the final operand  a 16bit immediate value.
The function addi
is given each of the operands for a MIPS addi instruction.
Add code so that it returns the encoded binary form of the instruction.
Your task is to add code to this function in addi.c:
// return the encoded binary MIPS for addi $t,$s, i
uint32_t addi(int t, int s, int i) {
return 42; // REPLACE WITH YOUR CODE
}
1521 spim2hex
can be used
to print the encoded binary form of a particular addi instruction as a hexadecimal, for example:
echo 'addi $9 $27 42'  1521 spim2hex 2369002a ./addi 9 27 42 addi(9, 27, 42) returned 0x2369002a echo 'addi $17 $19 3'  1521 spim2hex 2271fffd ./addi 17 19 3 addi(17, 19, 3) returned 0x2271fffdWith the first example above, we can see that the binary representation of
2369002a
is001000 11011 01001 0000000000101010
with the opcode being represented in the first group,
27
being represented by the
second group, 9
being represented by the third group, and the immediate value 42
being represented by the last group.
Use make(1) to build your code:
make # or 'make addi'
When you think your program is working,
you can use autotest
to run some simple automated tests:
1521 autotest addi
When you are finished working on this exercise,
you must
submit your work by running give
:
give cs1521 lab07_addi addi.c
You must run give
before Monday 28 October 12:00 (midday) (20241028 12:00:00)
to obtain the marks for this lab exercise.
Note that this is an individual exercise,
the work you submit with give
must be entirely your own.
Exercise — individual:
Convert from big to littleendian in MIPS
In the files for this lab,
you have been given mips_networking.s
,
a MIPS assembler program that reads one number and then prints it.
Add code to mips_networking.s
to make it equivalent to this C program:
// Reads a 4byte value and reverses the byte order, then prints it
#include <stdint.h>
#include <stdio.h>
#define BYTE_MASK 0xFF
int main(void) {
int32_t network_bytes;
scanf("%d", &network_bytes);
int32_t computer_bytes = 0;
uint32_t byte_mask = BYTE_MASK;
computer_bytes = (network_bytes & byte_mask) << 24;
computer_bytes = (network_bytes & (byte_mask << 8)) << 8;
computer_bytes = (network_bytes & (byte_mask << 16)) >> 8;
computer_bytes = (network_bytes & (byte_mask << 24)) >> 24;
printf("%d\n", computer_bytes);
return 0;
}
In other words, it should read one number, network_bytes, reverse the byte order inside it, and print the result.
The MIPS documentation guide provides a set of bitwise operations that will be useful for this.
For example:
1521 mipsy mips_networking.s 5 83886080 1521 mipsy mips_networking.s 255 16777216 1521 mipsy mips_networking.s 1 1 1521 mipsy mips_networking.s 2147483647 129
When you think your program is working,
you can use autotest
to run some simple automated tests:
1521 autotest mips_networking
When you are finished working on this exercise,
you must
submit your work by running give
:
give cs1521 lab07_mips_networking mips_networking.s
You must run give
before Monday 28 October 12:00 (midday) (20241028 12:00:00)
to obtain the marks for this lab exercise.
Note that this is an individual exercise,
the work you submit with give
must be entirely your own.
Exercise — individual:
Extract The Components of a Float
Floatingpoint Representation
Singleprecision floatingpoint numbers that follow the IEEE 754 standard have an internal structure that looks like this:
The value is determined as:
\[
{1}^{\text{sign}} \times \left(1 + \text{frac}\right) \times 2^{\text{exp}  127}
\]
The 32 bits in each float
are used as follows:

sign
is a single bit that indicates the number's sign. If set to 0, the number is positive; if set to 1, the number is negative.

exp
is an unsigned 8bit value (giving a range of \([0\cdots255]\)) which is interpreted as a value in the range \([127\cdots128]\) by subtracting 127 (the
bias
) from the stored 8bit value. It gives a multiplier for the fraction part (i.e., \( 2^{\text{exp}127} \)).
frac
is a value in the range \( [0\cdots1] \), determined using positional notation: \[ \frac{\text{bit}_{22}}{2^{1}} + \frac{\text{bit}_{21}}{2^{2}} + \frac{\text{bit}_{20}}{2^{3}} + \cdots + \frac{\text{bit}_{2}}{2^{21}} + \frac{\text{bit}_{1}}{2^{22}} + \frac{\text{bit}_{0}}{2^{23}} \] The overall value of the floatingpoint value is determined by adding 1 to the fraction: we assume that the "fraction" part is actually a value in the range \( [1\cdots2] \), but save bits by not explicitly storing the leading 1 bit.
For example:
raw 32 bits: 01000000001000000000000000000000 partitioned: 0 10000000 01000000000000000000000 sign: 0, so positive exp: 10000000 = 128, so multiplier is 2^{128127} = 2^{1} frac: 0×2^{1} + 1×2^{2} + ... = 0.25, but we need to add 1 final value: 1.25 × 2^{1} = 2.5
Exercise Description
Your task is to add code to this function in float_bits.c:
// separate out the 3 components of a float
float_components_t float_bits(uint32_t f) {
// PUT YOUR CODE HERE
}
The function float_bits
is given
the bits of a float as type uint32_t
.
Add code so that it returns a struct float_components
,
containing the sign, exponent and fraction fields of the struct
.
You also should add appropriate code to the functions
is_nan
,
is_positive_infinity
,
is_negative_infinity
, and
is_zero
.
All four functions take
a struct float_components
as their argument,
and return 0 or 1 depending on
some property of the float
it represents:
is_nan
returns 1 iff thestruct float_components
corresponds to the notanumber value, NaN, and 0 otherwise;is_positive_infinity
returns 1 iff thestruct float_components
corresponds to the positive infinity,inf
, and 0 otherwise;is_negative_infinity
returns 1 iff thestruct float_components
corresponds to the negative infinity,inf
, and 0 otherwise; andis_zero
returns 1 iff thestruct float_components
corresponds to either positive or negative zero.
These function must be implemented only using bit operations and integer comparisons.
Once these functions are completed, you should get output like:
./float_bits 42 float_bits(42) returned sign=0x1 exponent=0x84 fraction=0x280000 is_nan returned 0 is_positive_infinity returned 0 is_negative_infinity returned 0 is_zero returned 0 ./float_bits 3.14159 float_bits(3.14159012) returned sign=0x0 exponent=0x80 fraction=0x490fd0 is_nan returned 0 is_positive_infinity returned 0 is_negative_infinity returned 0 is_zero returned 0 ./float_bits inf float_bits(inf) returned sign=0x1 exponent=0xff fraction=0x000000 is_nan returned 0 is_positive_infinity returned 0 is_negative_infinity returned 1 is_zero returned 0
Use make(1) to build your code:
make # or 'make float_bits'
When you think your program is working,
you can use autotest
to run some simple automated tests:
1521 autotest float_bits
When you are finished working on this exercise,
you must
submit your work by running give
:
give cs1521 lab07_float_bits float_bits.c
You must run give
before Monday 28 October 12:00 (midday) (20241028 12:00:00)
to obtain the marks for this lab exercise.
Note that this is an individual exercise,
the work you submit with give
must be entirely your own.
Exercise — individual:
Multiply A Float by 2048 Using Bit Operations
Your task is to add code to this function in float_2048.c:
// float_2048 is given the bits of a float f as a uint32_t
// it uses bit operations and + to calculate f * 2048
// and returns the bits of this value as a uint32_t
//
// if the result is too large to be represented as a float +inf or inf is returned
//
// if f is +0, 0, +inf or inf, or Nan it is returned unchanged
//
// float_2048 assumes f is not a denormal number
//
uint32_t float_2048(uint32_t f) {
// PUT YOUR CODE HERE
return 42;
}
Add code to the function float_2048
so that, given a float as a uint32_t
,
it multiplies that value by 2048
using only bit operations and addition (+),
and returns the result.
If, after multiplication,
the result is too large to be represented as a float
,
return inf
if the original float
was positive,
and inf
if it was negative.
Once your program is working, you should see something like:
./float_2048 1 2048 ./float_2048 3.14159265 6433.98193 ./float_2048 2.718281828e20 5.56704133e17 ./float_2048 1e38 inf ./float_2048 1e37 inf ./float_2048 inf inf ./float_2048 inf inf ./float_2048 nan nan
Use make(1) to build your code:
make # or 'make float_2048'
When you think your program is working,
you can use autotest
to run some simple automated tests:
1521 autotest float_2048
When you are finished working on this exercise,
you must
submit your work by running give
:
give cs1521 lab07_float_2048 float_2048.c
You must run give
before Monday 28 October 12:00 (midday) (20241028 12:00:00)
to obtain the marks for this lab exercise.
Note that this is an individual exercise,
the work you submit with give
must be entirely your own.
Challenge Exercise — individual:
Compare Floats Using Bit Operations
Your task is to add code to this function in float_less.c:
// float_less is given the bits of 2 floats bits1, bits2 as a uint32_t
// and returns 1 if bits1 < bits2, 0 otherwise
// 0 is return if bits1 or bits2 is Nan
// only bit operations and integer comparisons are used
uint32_t float_less(uint32_t bits1, uint32_t bits2) {
// PUT YOUR CODE HERE
return 42;
}
Add code to the function float_less
,
which takes the bits of two floats
as the type uint32_t
,
and returns 1 if
the value of the first float
is less than the second,
and 0 otherwise.
It should do this using bit operations and integer comparisons.
For example:
./float_less 1 2 float_less(1, 2) returned 1 which is correct float_less(2, 1) returned 0 which is correct ./float_less 3.14159265 2.718281828e20 float_less(3.14159274, 2.7182819e20) returned 1 which is correct float_less(2.7182819e20, 3.14159274) returned 0 which is correct ./float_less inf inf float_less(inf, inf) returned 1 which is correct float_less(inf, inf) returned 0 which is correct ./float_less 0 0 float_less(0, 0) returned 0 which is correct float_less(0, 0) returned 0 which is correct ./float_less NaN inf float_less(nan, inf) returned 0 which is correct float_less(inf, nan) returned 0 which is correct
Use make(1) to build your code:
make # or 'make float_less'
When you think your program is working,
you can use autotest
to run some simple automated tests:
1521 autotest float_less
When you are finished working on this exercise,
you must
submit your work by running give
:
give cs1521 lab07_float_less float_less.c
You must run give
before Monday 28 October 12:00 (midday) (20241028 12:00:00)
to obtain the marks for this lab exercise.
Note that this is an individual exercise,
the work you submit with give
must be entirely your own.
Challenge Exercise — individual:
Print A Float Using Only putchar and puts
Your task is to add code to this function in float_print.c:
//
// float_print is given the bits of a float as a uint32_t
// it prints out the float in the same format as "%.9g\n"
// using only putchar & puts
//
void float_print(uint32_t bits) {
// PUT YOUR CODE HERE
}
The function float_print
takes a float as a uint32_t
.
Add code to it that prints its value
as a decimal, in "%.9g"
format,
using only putchar(3) or puts(3).
You cannot use any other functions,
such as printf(3).
Once your implementation is working, you should see things like:
./float_print 1 1 ./float_print 3.14159265 3.14159274 ./float_print 2.718281828e20 2.7182819e20 ./float_print 1.7e38 1.69999998e+38 ./float_print inf inf ./float_print inf inf ./float_print nan nan
Use make(1) to build your code:
make # or 'make float_print'
When you think your program is working,
you can use autotest
to run some simple automated tests:
1521 autotest float_print
When you are finished working on this exercise,
you must
submit your work by running give
:
give cs1521 lab07_float_print float_print.c
You must run give
before Monday 28 October 12:00 (midday) (20241028 12:00:00)
to obtain the marks for this lab exercise.
Note that this is an individual exercise,
the work you submit with give
must be entirely your own.
Submission
give
.
You can run give
multiple times.
Only your last submission will be marked.
Don't submit any exercises you haven't attempted.
If you are working at home, you may find it more convenient to upload your work via give's web interface.
Remember you have until Week 8 Monday 12:00:00 (midday) to submit your work without receiving a late penalty.
You cannot obtain marks by emailing your code to tutors or lecturers.
You check the files you have submitted here.
Automarking will be run by the lecturer several days after the submission deadline,
using test cases different to those autotest
runs for you.
(Hint: do your own testing as well as running autotest
.)
After automarking is run by the lecturer you can view your results here. The resulting mark will also be available via give's web interface.
Lab Marks
When all components of a lab are automarked you should be able to view the the marks via give's web interface or by running this command on a CSE machine:
1521 classrun sturec