ECS50-L25

📗 -> 11/27/24: ECS50-L25

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[Lecture Slide Link]

🎤 Vocab

❗ Unit and Larger Context

Final Review Topics

• It’s all bits
• Bitwise operators
• How many bits needed to represent
  • With B bits
• Unsigned, Sign magnitude, 2s complement
• Floatign point
• Hex
• Covnerting Between bases
• Structus
• Arrays, multidimensional
• Debugging
• Assembly / Intel cheat sheet
  • Advanced indexing (accessing memory)
    • leal
• GCC calling conventions
• Components of computer
  • Components of computer
• Why programs aren’t portable
• Inline assembly
• Stack and Stack frames
• Prologue and epilogue
• I/O
  • Polling vs interrupt driven I/O
  • Memory mapped vs I/O mapped
  • Interrupts
  • OS
    • What does it do
    • Applications loaded
    • Bootloader an how it loads the OS
    • Time sharing
• System calls
• Parallelism pipelining
  • Programmer: Message passing or shared memory
• Instructions are represented with bits
  • Internals of representation

✒️ -> Scratch Notes

Linux System Call Tables
int $0x80 system calls

Virtual Memory

• Notice how no matter how many times you run a program, you always have the same pointer values, the same ‘addresses’
• On creating an application, given virtual memory
• Allocated in ‘blocks’
  • Won’t go past block with a +1 error, only seg faults (going past allocated block) when you go significantly past your block

Printing From Assembly

fun(int a, int b, int c) {
	int x, y, z;

	p ((int *)$ebp)[2]@3  # print args
	p ((int *)$ebp)[-3]@3 # print locals
}

Midterm 2 Review

1. CPU instructions

100,000 * 4 / 50,000

4. Conflicting compiler instructions

jb/jae or jl/jge
Don't know how to compare signed and unsigned

5. Just push, pop, and call manually

praerielearn

6. 1 GDB command to print out the value before it returns

p $eax

7. Allocating space for program

same as praerielearn, instruction / data or text

8. Which parts are part of the stack

everything greater or equal than ESP pointing to

9. Save registers? Minimal set for callee

Caller:
- Save only ACD

Callee:
- Save everything but ACD

ESP is weird, left off the exam
- Consider it Callee saved

10. Write down the component name

Instruction - IR
Address of instruct - PC
Communicate - BUS
MAR
MDR
Arithmatetic - ALU
Stores running data - Memory
Points to top of stack - ESP

11. Whats in MAR and MDR?

movl %eax, %ebx: 
- MAR: No Value
- MDR: No Value

push %edx: 
- MAR: 996 (subtract 4 from 1000)
- MDR: 50(value pushed)

movl 400(%eax, %edx, 4), %ecx:
- MAR: 1600 (value computed)
- MDR: -786 (value at 1600)

ret
- MAR: 1000 (the memory popped from)
- MDR: 255 (the value popped)

movw $77, ($ebp)
- MAR: 200
- MDR: 77

pop %ecx
- 1000
- 255

movb (%esi, %ebx), %dh
- 3100 
- Unknown (past addresses shown)

Inline Assembly

bextr - Bit extraction?

bextr.cpp
 
unsigned int get_bits(int from, int start, int end) {
	int num_bits = end - start + 1;
	unsigned result;
	__asm__(
		//code
		"movb ch"
		"movb cl"
		"bextr %%ecx, %[from], %[result]"
		:
		//outputs
		[result] "=r" (result)
		:
		//inputs
		[from] "r" (from), [start]"b"(start), [amount] "a" (num_bits)
		:
		//clobbering
		"cc", "%ecx"
	);
}
 
int main() {
	unsigned int val = 0b011 1011 1101 1101;
	unsigned int bits = get_bits(val, 4, 9);
	printf("%x\n", bits); // prints 1D
	return 0;
}

🔗 -> Links

Resources

• Put useful links here

Connections

• Link all related words