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8c1491 Bob Green 2026-06-02 12:30:13
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# Runtime (crt0)
19a02c Bob Green 2026-06-02 12:15:40
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By default, when using my link scripts, the standard runtime is initialised by crt0 as follows:
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7d18d3 Bob Green 2026-06-02 12:57:34
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| Address | Size | Variable Name |Comment |
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| ------- | ------- | ------- | ------- |
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| `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 |
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| `0x040000` | n/a | `_RUN_ADDRESS` | Start of user program (.text), followed by initialised data (.data) and then unititialised data (.bss). |
aa3982 Bob Green 2026-06-02 12:27:17
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12cd8e Bob Green 2026-06-02 12:51:29
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You can override the values at link time by using the linker option:
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```
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--defsym=<Variable Name>=<value>
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```
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You can use this option as many times as needed.
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8b70b5 Bob Green 2026-06-02 13:03:34
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## Before calling main()
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### Banner Message
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If the code is running in ROM, a banner message will be printed, similar to the following:
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```
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Mega-680x0 Computer System
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Code is running in ROM
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68030 Processor running at ~40MHz
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```
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### Heap
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All free memory from the end of the .bss area to the end of ram to the heap (malloc/free).
018e16 Bob Green 2026-06-02 12:44:24
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### Global Variables
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crt0 sets the following global variables:
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| Variable | Comment |
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| ------- | ------- |
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| **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. |
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| **uint8_t cpu_type**. | Figures out what CPU is installed: 0 - 68000/68008, 1 - 68010, 2 - 68020, 3 - 68030 |
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| **uint8_t running_in_rom**. | When non-zero, indicates that the code is running in ROM. 0 indicates it's running in RAM. |
018e16 Bob Green 2026-06-02 12:44:24
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dcb313 Bob Green 2026-06-02 12:49:16
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To access any of these variables, simply ```#include <machine.h.>```
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### On Exit
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When `main()` returns or when `exit()` is called, a non-zero exit code will be displayed as follows:
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```
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exited with status: <value>
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```
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No message is printed if the exit code was zero.
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Control will then be passed back to the monitor