Memory & Pointers 1
CMPT-225, Fall 2021
Computer Memory
- A sequence of locations
- Indexed by address: 0,1,2,…
- Each location stores a data byte
- Processor can read or write the byte at each address.
- Regions of memory are allocated to processes as needed, according to some scheme.
Diagram displaying “Code + Data for running part of OS” at the start of memory, “Free memory” in the middle, and “Code + Data for running user processes” and the end.
Variables & Memory
A variable is (roughly) and names & tagged collection of bytes:
Diagram showing:
- “int x; x = 6” taking 4 bytes of memory,
- “char c; c = ‘x’;” taking 1 byte of memory
-
“bool b; b = ‘true’;” taking 1 byte of memory
- So, at run time, each variable has an address in memory.
- In C, C++ we can:
- access the address of a variable
- access a variable or memory location by its address
- declare variables by storing addresses (pointers).
Addresses & Pointers - By Example
- “int i = 5;”
- allocate space for an int,
- name the space “i”,
- store 5 there
- “int *p;”
- allocate space for an address,
- name it p,
- record its type as “pointer to int”
- “p = ∧i;”
- “&i” is the address of “i”
- store ∧i in p
- so, p becomes a pointer to i
- “cout « i;”
- outputs the value stored in i, 5
- “cout « p;”
- outputs the address of i: 0xbffffbbc
- “cout « *p;”
- ”*” dereferences p. That is, *p is the value pointed to by p. In this case 5
Diagram of memory:
| Address | Name | Value |
| … | … | … |
| 254 | i | 5 |
| … | … | … |
| 923 | p | 254 |
Second diagram of memory:
| Address | Name | Value | Size |
| 0x6fffbe | i | 5 | 4 bytes |
| … | … | … | … |
| ??? | p | 0x6fffbe | 4 bytesa |
Slide showing program code: see pointers.c
Slide showing output: see output.txt
Slide showing same program with different highlights.
Slide showing same output with different highlights.