blog

COMP1521 Week 2 Data

This week is about data.

make and Makefile

Make (build system) is a powerful tool.

bm : bm.o Stack.o
	gcc -o bm bm.o Stack.o

bm.o : bm.c Stack.h
	gcc -c -Wall -Werror bm.c

Stack.o : Stack.c Stack.h
	gcc -c -Wall -Werror Stack.c

There are three components: target, source and action. Words before : is the target, after it is the source, and below it is the action.
make is smart as it compares the modification timestamp of sources to decide what actions to do (if it is newly modified).

Memory

Memory regions during C program execution

Physical Memory

• called main memory (or RAM, or primary storage, …)
• indexes are “memory addresses” (a.k.a. pointers)
• data can be fetched in chunks of 1,2,4,8 bytes
• cost of fetching any byte is same (ns)
• can be volatile or non-volatile
• When addressing objects in memory, any byte address can be used to fetch 1-byte object, and byte address for N-byte object must be divisible by N

  big-endian vs little-endian

  

Data Representation

Memory allows you to load bit-strings of sizes 1,2,4,8 bytes from N-byte boundary addresses into registers in the CPU. Data representation is to give these bytes meanings.

Character Data

Character Data is about encodings of characters.

ASCII (ISO 646)

• 7-bit values, using lower 7-bits of a byte (top bit always zero)
• can encode roman alphabet, digits, punctuation, control chars
• Uses values in the range 0x00 to 0x7F (0..127)
  • control characters (0..31) … e.g. ‘\0’, ‘\n’
  • punctuation chars (32..47,91..96,123..126)
  • digits (48..57) … ‘0’..’9’
  • upper case alphabetic (65..90) … ‘A’..’Z’
  • lower case alphabetic (97..122) … ‘a’..’z’

    UTF-8 (Unicode)

• UTF-8 is one of the three standard character encodings used to represent Unicode as computer text (the others being UTF-16 and UTF-32) reference
• 8-bit values, with ability to extend to multi-byte values
• can encode all human languages plus other symbols
  • The 127 1-byte codes are compatible with ASCII
  • The 2048 2-byte codes include most Latin-script alphabets
  • The 65536 3-byte codes include most Asian languages
  • The 2097152 4-byte codes include symbols and emojis

Numeric Data

Numeric data comes in two major forms: integer and floating point numbers

Signed Integers

• signed magnitude : first bit is sign, rest are magnitude
• ones complement : form -N by inverting all bits in N
• twos complement : form -N by inverting N and adding 1, very useful for computer

  Floating Point Numbers

  
  single vs double
  IEEE 754 Standard for Floating-Point Arithmetic
  Double is more precise than single and occupies more memory.

Fun stuffs

For week 2’s lab, we are to create a struct for big number and write a big number adder. One of the challenge task and one of the most interesting task is to make the struct more compact. At first, we used unsigned char to represent a digit, and I was thinking of to use a data type that occupies only 4 bits (enough to represent 1 - 16). But after searching around, I found that unsigned char or uinit8_t is the smallest data types in C, which only occupies 1 byte, which is atomic.