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267 lines
9.0 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Crt0 (Embed with GNU)</title>
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<meta name="description" content="Crt0 (Embed with GNU)">
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<meta name="keywords" content="Crt0 (Embed with GNU)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="leds_002ec.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Libraries.html#Libraries" rel="up" title="Libraries">
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<link href="Linker-Scripts.html#Linker-Scripts" rel="next" title="Linker Scripts">
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<link href="Libraries.html#Libraries" rel="prev" title="Libraries">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Crt0"></a>
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<div class="header">
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<p>
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Next: <a href="Linker-Scripts.html#Linker-Scripts" accesskey="n" rel="next">Linker Scripts</a>, Up: <a href="Libraries.html#Libraries" accesskey="u" rel="up">Libraries</a> [<a href="leds_002ec.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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</div>
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<hr>
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<a name="Crt0_002c-the-main-startup-file"></a>
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<h3 class="section">3.1 Crt0, the main startup file</h3>
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<p>To make a program that has been compiled with GCC to run, you
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need to write some startup code. The initial piece of startup code is
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called a crt0. (C RunTime 0) This is usually written in assembler, and
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it’s object gets linked in first, and bootstraps the rest of the
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application when executed. This file needs to do the following things.
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</p>
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<ol>
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<li> Initialize anything that needs it. This init section varies. If you are
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developing an application that gets download to a ROM monitor, then
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there is usually no need for any special initialization. The ROM monitor
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handles it for you.
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<p>If you plan to burn your code in a ROM, then the crt0 typically has to
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do all the hardware initialization that is required to run an
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application. This can include things like initializing serial ports or
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run a memory check. It all depends on the hardware.
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</p>
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</li><li> Zero the BSS section. This is for uninitialized data. All the addresses in
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this section need to be initialized to zero so that programs that forget
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to check new variables default value will get unpredictable results.
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</li><li> Call main()
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This is what basically starts things running. If your ROM monitor
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supports it, then first setup argc and argv for command line arguments
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and an environment pointer. Then branch to main(). For G++ the the main
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routine gets a branch to __main inserted by the code generator at the
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very top. __main() is used by G++ to initialize it’s internal tables.
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__main() then returns back to your original main() and your code gets
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executed.
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</li><li> Call exit()
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After main() has returned, you need to cleanup things and return control
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of the hardware from the application. On some hardware, there is nothing
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to return to, especially if your program is in ROM. Sometimes the best
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thing to do in this case is do a hardware reset, or branch back to the
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start address all over again.
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<p>When there is a ROM monitor present, usually a user trap can be called
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and then the ROM takes over. Pick a safe vector with no side
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effects. Some ROMs have a builtin trap handler just for this case.
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</p></li></ol>
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<p>portable between all the m68k based boards we have here.
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<a href="crt0_002eS.html#crt0_002eS">Example Crt0.S</a>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/* ANSI concatenation macros. */
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#define CONCAT1(a, b) CONCAT2(a, b)
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#define CONCAT2(a, b) a ## b
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</pre></div>
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<p>These we’ll use later.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/* These are predefined by new versions of GNU cpp. */
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#ifndef __USER_LABEL_PREFIX__
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#define __USER_LABEL_PREFIX__ _
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#endif
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/* Use the right prefix for global labels. */
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#define SYM(x) CONCAT1 (__USER_LABEL_PREFIX__, x)
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</pre></div>
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<p>These macros are to make this code portable between both <em>COFF</em> and
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<em>a.out</em>. <em>COFF</em> always has an <var>_ (underline)</var> prepended on
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the front of all global symbol names. <em>a.out</em> has none.
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</p>
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<div class="smallexample">
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<pre class="smallexample">#ifndef __REGISTER_PREFIX__
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#define __REGISTER_PREFIX__
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#endif
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/* Use the right prefix for registers. */
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#define REG(x) CONCAT1 (__REGISTER_PREFIX__, x)
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#define d0 REG (d0)
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#define d1 REG (d1)
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#define d2 REG (d2)
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#define d3 REG (d3)
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#define d4 REG (d4)
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#define d5 REG (d5)
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#define d6 REG (d6)
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#define d7 REG (d7)
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#define a0 REG (a0)
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#define a1 REG (a1)
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#define a2 REG (a2)
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#define a3 REG (a3)
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#define a4 REG (a4)
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#define a5 REG (a5)
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#define a6 REG (a6)
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#define fp REG (fp)
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#define sp REG (sp)
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</pre></div>
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<p>This is for portability between assemblers. Some register names have a
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<var>%</var> or <var>$</var> prepended to the register name.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/*
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* Set up some room for a stack. We just grab a chunk of memory.
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*/
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.set stack_size, 0x2000
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.comm SYM (stack), stack_size
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</pre></div>
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<p>Set up space for the stack. This can also be done in the linker script,
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but it typically gets done here.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/*
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* Define an empty environment.
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*/
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.data
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.align 2
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SYM (environ):
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.long 0
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</pre></div>
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<p>Set up an empty space for the environment. This is bogus on any most ROM
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monitor, but we setup a valid address for it, and pass it to main. At
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least that way if an application checks for it, it won’t crash.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> .align 2
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.text
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.global SYM (stack)
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.global SYM (main)
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.global SYM (exit)
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/*
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* This really should be __bss_start, not SYM (__bss_start).
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*/
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.global __bss_start
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</pre></div>
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<p>Setup a few global symbols that get used elsewhere. <var>__bss_start</var>
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needs to be unchanged, as it’s setup by the linker script.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/*
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* start -- set things up so the application will run.
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*/
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SYM (start):
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link a6, #-8
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moveal #SYM (stack) + stack_size, sp
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/*
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* zerobss -- zero out the bss section
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*/
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moveal #__bss_start, a0
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moveal #SYM (end), a1
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1:
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movel #0, (a0)
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leal 4(a0), a0
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cmpal a0, a1
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bne 1b
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</pre></div>
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<p>The global symbol <code>start</code> is used by the linker as the default
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address to use for the <code>.text</code> section. then it zeros the
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<code>.bss</code> section so the uninitialized data will all be cleared. Some
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programs have wild side effects from having the .bss section let
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uncleared. Particularly it causes problems with some implementations of
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<code>malloc</code>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/*
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* Call the main routine from the application to get it going.
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* main (argc, argv, environ)
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* We pass argv as a pointer to NULL.
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*/
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pea 0
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pea SYM (environ)
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pea sp@(4)
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pea 0
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jsr SYM (main)
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movel d0, sp@-
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</pre></div>
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<p>Setup the environment pointer and jump to <code>main()</code>. When
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<code>main()</code> returns, it drops down to the <code>exit</code> routine below.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/*
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* _exit -- Exit from the application. Normally we cause a user trap
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* to return to the ROM monitor for another run.
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*/
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SYM (exit):
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trap #0
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</pre></div>
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<p>Implementing <code>exit</code> here is easy. Both the <code>rom68k</code> and <code>bug</code>
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can handle a user caused exception of <code>zero</code> with no side effects.
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Although the <code>bug</code> monitor has a user caused trap that will return
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control to the ROM monitor, this solution has been more portable.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Linker-Scripts.html#Linker-Scripts" accesskey="n" rel="next">Linker Scripts</a>, Up: <a href="Libraries.html#Libraries" accesskey="u" rel="up">Libraries</a> [<a href="leds_002ec.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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</div>
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</body>
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</html>
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