Intercept machine.mem32 etc. (#78)

Provide a minimal implementation that allows reading the device ID but otherwise raises an exception.

I'm open to adding other FICR but this is the priority to get the code on this support article:
https://support.microbit.org/support/solutions/articles/19000070728-how-to-find-the-micro-bit-serial-number
...working in the simulator.

Closes #58

Author: microbit-matt-hillsdon

src/Makefile

@@ -60,6 +60,7 @@ SRC_C += \
 	microbithal_js.c \
 	main.c \
 	mphalport.c \
+	modmachine.c \
 
 SRC_C += $(addprefix $(CODAL_PORT)/, \
 	drv_display.c \

src/modmachine.c

@@ -0,0 +1,34 @@
+#include "py/runtime.h"
+
+// Just defines the memory access functions, otherwise we use codal_port's
+// implementation.
+
+uintptr_t machine_mem_get_read_addr(mp_obj_t addr_o, uint align) {
+    uintptr_t addr = mp_obj_get_int_truncated(addr_o);
+        if ((addr & (align - 1)) != 0) {
+        mp_raise_msg_varg(&mp_type_ValueError, MP_ERROR_TEXT("address %08x is not aligned to %d bytes"), addr, align);
+    }
+
+    static const uint32_t FICR = 0x10000000;
+    static const uint32_t FICR_DEVICEID_0 = FICR + 0x060;
+    static const uint32_t FICR_DEVICEID_1 = FICR + 0x064;
+
+    static uint32_t mem;
+    switch (addr) {
+        case FICR_DEVICEID_0:
+        case FICR_DEVICEID_1: {
+            // There's machine.unique_id backed by hal for this
+            // but existing code reads via FICR.
+            mem = 0;
+            break;
+        }
+        default: {
+            mp_raise_NotImplementedError(MP_ERROR_TEXT("simulator limitation: memory read"));
+        }
+    }
+    return (uintptr_t)&mem;
+}
+
+uintptr_t machine_mem_get_write_addr(mp_obj_t addr_o, uint align) {
+    mp_raise_NotImplementedError(MP_ERROR_TEXT("simulator limitation: memory write"));
+}

src/mpconfigport.h

@@ -158,4 +158,8 @@ typedef long mp_off_t;
 // Needed for MICROPY_PY_URANDOM_SEED_INIT_FUNC.
 extern uint32_t rng_generate_random_word(void);
 
+// Intercept modmachine memory access.
+#define MICROPY_MACHINE_MEM_GET_READ_ADDR machine_mem_get_read_addr
+#define MICROPY_MACHINE_MEM_GET_WRITE_ADDR machine_mem_get_write_addr
+
 #endif