Define heap in linker and place other sections correctly

linker.ld

@@ -1,15 +1,40 @@
 ENTRY(_start)
 
+MEMORY
+{
+    FLASH  (rx)  : ORIGIN = 0x00000000, LENGTH = 1M
+    RAM  (rwx) : ORIGIN = 0x20000000, LENGTH = 128K
+}
+
+_Min_Heap_Size = 0x200 ;
+_Min_Stack_Size = 0x200 ;       /* Required amount of stack.  Used by main(), then re-used as the interrupt stack after the kernel starts. */
+_estack = ORIGIN(RAM) + LENGTH(RAM);
+
+
 SECTIONS
 {
-    /* text section (code)*/
+	.text : { *(.text*) } 		> FLASH
-    .text : { *(.text*) }
+	.rodata  : { *(.rodata*) }	> FLASH
-    /* data section, initialized variables */
+	.bss  : { *(.bss*) }		> RAM
-    .data : { *(.data) }
+	.data  : { *(.data*) }		> RAM
-    ram_start = 0x20000000;
+	.heap : {
-    /* bss section, uninitialized variables */
+		. = ALIGN(8);
-    .bss : { *(.bss*) }
+        PROVIDE ( end = . );
-    /* stack section */
+        PROVIDE ( _end = . );
-    /* The stack is placed at the end of the RAM */
+        _heap_bottom = .;
-    stack_top = 0x2001ffff;
+        . = . + _Min_Heap_Size;
+        _heap_top = .;
+        . = . + _Min_Stack_Size;
+		. = ALIGN(8);
+	} >RAM
+
+	.stack (NOLOAD) :  {
+		. = ORIGIN(RAM) + LENGTH(RAM);
+		stack_top = .;
+	} > RAM
+
+	__StackLimit = stack_top - _Min_Stack_Size;
+
+	/* Check if data + heap + stack exceeds RAM limit */
+	ASSERT(__StackLimit >= _heap_top, "region RAM overflowed with stack")
 }

os/alloc.c

@@ -1,14 +1,16 @@
 #include "alloc.h"
 #include "stdint.h"
 
-void* *first_mem_avail = (void*) 0x20000000;
+HeapArena heap_arena;
+
+extern void * _heap_top;
 
 void init_allocator() {
-    *first_mem_avail = (void*) 0x20000000 +sizeof(void*);
+    heap_arena.wilderness = &_heap_top;
 }
 
 void* malloc(size_t size) {
-    void* block = *first_mem_avail;
+    void* block = heap_arena.wilderness;
-    *first_mem_avail += size;
+    heap_arena.wilderness += size;
     return block;
 }

os/alloc.h

@@ -1,4 +1,9 @@
 #include <stddef.h>
+#include <stdint.h>
+
+typedef struct {
+    void * wilderness;
+} HeapArena;
 
 void init_allocator();
 void* malloc (size_t size);