Exams/AdvOpsys/notes/work-summary-2026-04-14.md

Assignment 4 – Part 1: Memory Management

Overview

In this part of the assignment, a basic memory management system was implemented for a simple OS kernel. The goal was to initialize kernel memory, enable paging, and implement dynamic memory allocation using malloc() and free().

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Implemented Features

Kernel Memory Initialization

• Used:

  extern uint32_t end;
  

  to get the end address of the kernel from the linker.
• Implemented:

  InitKernelMemory(&end);
  

• Set up:
  • heap_begin
  • heap_end
  • pheap_begin (page-aligned heap)
  • pheap_end

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Paging

• Implemented:

  InitPaging();
  

• Set up:
  • Page directory at 0x400000
  • Page tables starting at 0x404000
• Identity-mapped:
  • 0x00000000 → 0x00000000
  • 0x400000 → 0x400000
• Enabled paging by modifying cr0 and cr3.

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Dynamic Memory Allocation

malloc()

• Implemented a simple heap allocator.
• Stores metadata using:

  typedef struct {
      uint8_t status;
      uint32_t size;
  } alloc_t;
  

• Supports:
  • Sequential allocation
  • Reuse of freed blocks
• Zeroes allocated memory using memset.

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free()

• Frees memory by marking the block as unused.
• Prevents errors by:

  if (!mem) return;
  

• Correctly updates memory_used.

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Block Reuse

• When a block is freed, it can be reused by future malloc() calls.
• Verified by:
  • Freeing a block
  • Allocating a smaller block
  • Observing same address reused

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Page-Aligned Allocation (pmalloc)

• Implemented pmalloc() and pfree():
  • Allocates memory in 4KB pages
  • Uses a descriptor array (pheap_desc)
• Located in upper memory region near 0x400000

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Memory Utility Functions

Implemented:

• memcpy()
• memset()
• memset16()

These replace standard library functions in kernel space.

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Debug Output

Memory Layout

• Implemented:

  PrintMemoryLayout();
  

• Displays:
  • Memory used
  • Memory free
  • Heap size
  • Heap start/end
  • Page heap start/end

Allocation Logs

• Outputs allocation details:

  Allocated X bytes from 0x... to 0x...
  Re-allocated X bytes from 0x... to 0x...
  

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Hex Output for Addresses

• Implemented:

  TerminalWriteHex(uint32_t num);
  

• Ensures correct formatting of memory addresses (base 16)
• Avoids confusion from decimal output

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Testing & Verification

Allocation Test

void* memory1 = malloc(48261);
void* memory2 = malloc(27261);
void* memory3 = malloc(12617);

Free and Reuse Test

free(memory2);
void* memory4 = malloc(1000);

Result

• memory4 reused the same address as memory2
• Confirms:
  • free() works
  • allocator reuses memory correctly

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Conclusion

• Successfully implemented a basic kernel memory manager
• Paging is enabled and functioning
• malloc() and free() work correctly
• Memory reuse is verified
• Debug output is clear and correctly formatted

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Next Step

Proceed to Part 2: PIT and Sleep Functions