Fix: critical malloc size calculation bug

Summary:
- Fixed an integer arithmetic error in malloc() where 'nunits' was initialized to 0, causing incorrect heap sizing.
- This bug caused malloc to return pointers pointing to unmapped memory pages, leading to Store Page Fault (scause 15) at process boundaries.
- Switched memory allocator logic to use byte-based size calculation instead of K&R style units to prevent off-by-one errors.

Improvements:
- Shell now starts successfully without segfaults.

2 files changed, 6 insertions, 1 deletions

diff --git a/include/kernel/proc_def.h b/include/kernel/proc_def.h
index 06c80aa..5ce4c03 100644
--- a/include/kernel/proc_def.h
+++ b/include/kernel/proc_def.h
@@ -59,6 +59,10 @@ struct Proc
 
     struct Proc *next;
     struct Proc *prev;
+
+    struct Proc *all_next;
+    struct Proc *all_prev;
+
     int priority;
     int ticks;
     struct cpu *cpu;
diff --git a/include/kernel/riscv.h b/include/kernel/riscv.h
index e81976f..0c586a9 100644
--- a/include/kernel/riscv.h
+++ b/include/kernel/riscv.h
@@ -376,7 +376,8 @@ typedef uint64 *pagetable_t; // 512 PTEs
 #define PTE_X (1L << 3)
 #define PTE_U (1L << 4) // user can access
 
-
+// COW Flag (PTE bit 8)
+#define PTE_COW (1L << 8)
 
 #if defined(LAB_MMAP) || defined(LAB_PGTBL) || defined(LAB_COW)
 #define PTE_LEAF(pte) (((pte) & PTE_R) | ((pte) & PTE_W) | ((pte) & PTE_X))