CS2100 Common Mistakes Part 1

“We learn from failure, not from success!”

— Bram Stoker

As you read this blog, you may realize that I am taking the CS 2K Trinity Modules, which are not commonly recommended. Therefore, I also need a system to manage the overwhelming content of these modules. While CS2030S is known for its heavy workload, CS2040S is known for its difficult concepts, and CS2100 is known for covering a broad range of topics but lacking depth. With this vast amount of knowledge, I am trying to note some of my misconceptions here so that I won’t repeat them.

Note: Part 1 covers the first half of the semester (before the midterm and recess week). I followed this convention in a previous post on CS2040S, but I think there isn’t an obvious separation for this course.

Lecture 3: Data Representation and Number Systems

When converting a binary number to an 8-bit 1’s or 2’s complement, such as 01001010, the result is simply 01001010 for both. You only negate the bits or add one after negation (for the 2’s complement) when you need to find the negative value -(01001010), which, for example, is 10110110.
The IEEE-754 format can be more conveniently represented as 8 hexadecimal digits, but it is worth noting that you first need to convert the binary into a form like $1.XXXX \times 2^n$. The 32 bits of IEEE-754 consist of 1 sign bit (0 for positive and 1 for negative), 8 bits for the exponent in excess-127 notation, and the remaining 23 bits represent the fractional part, arranged from left to right.

Below is a useful bit table to memorize or refer to, making binary-to-hexadecimal conversion faster:

Binary	Hexadecimal	Binary	Hexadecimal
0000	0	1000	8
0001	1	1001	9
0010	2	1010	A
0011	3	1011	B
0100	4	1100	C
0101	5	1101	D
0110	6	1110	E
0111	7	1111	F

Lecture 4: Pointers and Functions

We can declare a variable int *p, which is a pointer p pointing to an integer value. Likewise, int **q is a double pointer. Here, q is a pointer that points to a pointer that points to an integer value. For example, we can assign q = &p. The tricky part is when we dereference the double pointer. *q will point to p, but **q will point to the value that p is pointing to.

Lecture 5: Arrays, Strings, and Structures

In C, the function passes a value instead of a reference like in many other languages, so any variables passed in are local within the function scope, except for pointers.
The variable name of an array (for example, a) is a pointer to the first element of the array. Using this property, we can modify the structure in the first element of the array by passing the array name to the function.
Strings are just arrays of characters that end with a null terminator \0. This terminator is not shown when printed, but it is worth noting that the behavior of strings is similar to that of arrays.

Lecture 7: MIPS I (Introduction)

Every line in assembly code corresponds one-to-one with a line in the binary code.
Program variables are assigned to registers by compilers, not the programmers.
Registers don’t have data types like memory, and assembly could be considered a weakly typed language in this course.
The MIPS processor has 32 registers, each 32 bits wide, while the immediate operand (constant) in the addi instruction is a 16-bit two’s complement value (ranging from $-2^{15}$ to $2^{15} - 1$).
Shifting $n$ bits left or right multiplies or divides bit values by $2^n$ (not to be confused with just $n$). If the results from bit shifting are zero, it could be due to the original bits being zeros or the shift size being at least the number of bits.
Below are some useful instructions for performing operations that are not explicitly in the instruction set:
- sll for multiplication
- srl for division
- and for masking
- or for setting bits to 1
- nor/xor for not
A tip for adding a large constant (more than 16 bits for an immediate operand) is to use the lui instruction to set the first 16 bits (initializing the rest as zero), then use ori on that register to set the last 16 bits (while leaving the first 16 bits unchanged).

Lecture 8: MIPS II (More Instructions)

MIPS memory is 32 bits wide, allowing the processor to address $2^{32}$ locations, each holding 1 byte of data. Since words are 32 bits wide, each consecutive word occupies 4 addresses (4 bytes) and must be aligned.
The lw (load word) instruction reads values from right to left, while the sw (store word) instruction writes values from left to right.
The slt (set if less than) instruction sets the value to 1 if the condition is satisfied and to 0 otherwise.
Labels are not instructions; they are pointers and do not consume memory.
Be aware that the final loop iteration will also execute to check the condition, so the number of executed instructions is often off by one.

Lecture 9: MIPS III (Instruction Formats and Encoding)

To be updated.

Lecture 10: Instruction Set Architecture

To be updated.

Lecture 11: Processor: Datapath

To be updated.

Lecture 12: Processor: Control

To be updated.

CS2040S Probem Sets Part 1

MA1522 Common Mistakes