# Runtime (crt0)

By default, when using my link scripts, the standard runtime is initialised by crt0 as follows:

| Address | Size | Variable Name |Comment |
| ------- | ------- | ------- | ------- |
| `0x03fffe` | ~256k  | `_STACK_SIZE` |Stack (SSP). Grows towards 0. At round about 0x0000400 it would start stomping all over the memory area used by the monitor |
| `0x040000` | n/a    | `_RUN_ADDRESS` | Start of user program (.text), followed by initialised data (.data) and then unititialised data (.bss).  |

You can override the values at link time by using the linker option:

```
--defsym=<Variable Name>=<value>
```
You can use this option as many times as needed.

## Before calling main()
### Banner Message
If the code is running in ROM, a banner message will be printed, similar to the following:
```
Mega-680x0 Computer System
Code is running in ROM
68030 Processor running at ~40MHz
```

### Heap
All free memory from the end of the .bss area to the end of ram to the heap (malloc/free).

### Global Variables
crt0 sets the following global variables:

| Variable | Comment |
| ------- | ------- |
| **uint8_t cpu_speed_mhz**. | This uses the counter in the PI/T to figure out roughly how fast the CPU is being clocked at. Within a MHz or so. |
| **uint8_t cpu_type**. | Figures out what CPU is installed: 0 - 68000/68008, 1 - 68010, 2 - 68020, 3 - 68030 |
| **uint8_t running_in_rom**. | When non-zero, indicates that the code is running in ROM. 0 indicates it's running in RAM. |

To access any of these variables, simply ```#include <machine.h.>```

### On Exit
When `main()` returns or when `exit()` is called, a non-zero exit code will be displayed as follows:
```
exited with status: <value>
```
No message is printed if the exit code was zero.
Control will then be passed back to the monitor