Assignment 4 p1 completed

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

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+# 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()`.
+
+---
+
+## Implemented Features
+
+### Kernel Memory Initialization
+- Used:
+  ```c
+  extern uint32_t end;
+  ```
+  to get the end address of the kernel from the linker.
+- Implemented:
+  ```c
+  init_kernel_memory(&end);
+  ```
+- Set up:
+  - `heap_begin`
+  - `heap_end`
+  - `pheap_begin` (page-aligned heap)
+  - `pheap_end`
+
+---
+
+### Paging
+- Implemented:
+  ```c
+  init_paging();
+  ```
+- Set up:
+  - Page directory at `0x400000`
+  - Page tables starting at `0x404000`
+- Identity-mapped:
+  - `0x00000000 → 0x00000000`
+  - `0x400000 → 0x400000`
+- Enabled paging by modifying `cr0` and `cr3`.
+
+---
+
+### Dynamic Memory Allocation
+
+#### malloc()
+- Implemented a simple heap allocator.
+- Stores metadata using:
+  ```c
+  typedef struct {
+      uint8_t status;
+      uint32_t size;
+  } alloc_t;
+  ```
+- Supports:
+  - Sequential allocation
+  - Reuse of freed blocks
+- Zeroes allocated memory using `memset`.
+
+---
+
+#### free()
+- Frees memory by marking the block as unused.
+- Prevents errors by:
+  ```c
+  if (!mem) return;
+  ```
+- Correctly updates `memory_used`.
+
+---
+
+#### 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
+
+---
+
+### 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`
+
+---
+
+### Memory Utility Functions
+Implemented:
+- `memcpy()`
+- `memset()`
+- `memset16()`
+
+These replace standard library functions in kernel space.
+
+---
+
+### Debug Output
+
+#### Memory Layout
+- Implemented:
+  ```c
+  print_memory_layout();
+  ```
+- 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...
+  ```
+
+---
+
+### Hex Output for Addresses
+- Implemented:
+  ```c
+  TerminalWriteHex(uint32_t num);
+  ```
+- Ensures correct formatting of memory addresses (base 16)
+- Avoids confusion from decimal output
+
+---
+
+## Testing & Verification
+
+### Allocation Test
+```c
+void* memory1 = malloc(48261);
+void* memory2 = malloc(27261);
+void* memory3 = malloc(12617);
+```
+
+### Free and Reuse Test
+```c
+free(memory2);
+void* memory4 = malloc(1000);
+```
+
+### Result
+- `memory4` reused the same address as `memory2`
+- Confirms:
+  - `free()` works
+  - allocator reuses memory correctly
+
+---
+
+## 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
+
+---
+
+## Next Step
+Proceed to **Part 2: PIT and Sleep Functions**