diff --git a/py/gc.c b/py/gc.c
index 70b071ebc8..73e2436655 100644
--- a/py/gc.c
+++ b/py/gc.c
@@ -187,6 +187,22 @@ void gc_collect_end(void) {
     gc_sweep();
 }
 
+extern void gc_helper_get_regs_and_clean_stack(machine_uint_t *regs, machine_uint_t heap_end);
+
+void gc_collect(void) {
+    extern char _ram_start;  /* defined by linker script */
+    extern char _ram_end;    /* defined by linker script */
+    extern char _heap_start; /* defined by linker script */
+    extern char _heap_end;   /* defined by linker script */
+
+    gc_collect_start();
+    gc_collect_root((void**)&_ram_start, (&_heap_start - &_ram_start) / 4);
+    machine_uint_t regs[10];
+    gc_helper_get_regs_and_clean_stack(regs, (uint32_t) &_heap_end);
+    gc_collect_root((void**)&_heap_end, (&_ram_end - &_heap_end) / 4); // will trace regs since they now live in this function on the stack
+    gc_collect_end();
+}
+
 void gc_info(gc_info_t *info) {
     info->total = (gc_pool_end - gc_pool_start) * sizeof(machine_uint_t);
     info->used = 0;
diff --git a/stm/main.c b/stm/main.c
index 2ce2b6dc61..389fea2634 100644
--- a/stm/main.c
+++ b/stm/main.c
@@ -48,6 +48,7 @@
 int errno;
 
 extern uint32_t _heap_start;
+extern uint32_t _heap_end;
 
 static FATFS fatfs0;
 
@@ -455,21 +456,12 @@ bool do_file(const char *filename) {
     }
 }
 
-#define RAM_START (0x20000000) // fixed for chip
-#define HEAP_END  (0x2001c000) // tunable
-#define RAM_END   (0x20020000) // fixed for chip
+mp_obj_t pyb_gc(void) {
+    uint32_t start,ticks;
 
-void gc_helper_get_regs_and_clean_stack(machine_uint_t *regs, machine_uint_t heap_end);
-
-void gc_collect(void) {
-    uint32_t start = sys_tick_counter;
-    gc_collect_start();
-    gc_collect_root((void**)RAM_START, (((uint32_t)&_heap_start) - RAM_START) / 4);
-    machine_uint_t regs[10];
-    gc_helper_get_regs_and_clean_stack(regs, HEAP_END);
-    gc_collect_root((void**)HEAP_END, (RAM_END - HEAP_END) / 4); // will trace regs since they now live in this function on the stack
-    gc_collect_end();
-    uint32_t ticks = sys_tick_counter - start; // TODO implement a function that does this properly
+    start = sys_tick_counter;
+    gc_collect();
+    ticks = sys_tick_counter - start; // TODO implement a function that does this properly
 
     if (0) {
         // print GC info
@@ -480,10 +472,7 @@ void gc_collect(void) {
         printf(" %lu : %lu\n", info.used, info.free);
         printf(" 1=%lu 2=%lu m=%lu\n", info.num_1block, info.num_2block, info.max_block);
     }
-}
 
-mp_obj_t pyb_gc(void) {
-    gc_collect();
     return mp_const_none;
 }
 
@@ -609,7 +598,7 @@ int main(void) {
 soft_reset:
 
     // GC init
-    gc_init(&_heap_start, (void*)HEAP_END);
+    gc_init(&_heap_start, &_heap_end);
 
     // Micro Python init
     qstr_init();
diff --git a/stm/stm32f405.ld b/stm/stm32f405.ld
index c19e6a1c19..231dbcbe70 100644
--- a/stm/stm32f405.ld
+++ b/stm/stm32f405.ld
@@ -19,6 +19,10 @@ _minimum_heap_size = 16K;
 /* top end of the stack */
 _estack = ORIGIN(RAM) + LENGTH(RAM);
 
+_ram_start = 0x20000000;
+_ram_end   = 0x20020000;
+_heap_end  = 0x2001c000;
+
 /* define output sections */
 SECTIONS
 {