163 lines
5.4 KiB
HTML
163 lines
5.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
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<TITLE>Linux 2.4.x Initialization for IA-32 HOWTO: Linux architecture-specific initialization</TITLE>
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<LINK HREF="Linux-Init-HOWTO-5.html" REL=next>
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<LINK HREF="Linux-Init-HOWTO-3.html" REL=previous>
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<LINK HREF="Linux-Init-HOWTO.html#toc4" REL=contents>
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</HEAD>
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<BODY>
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<A HREF="Linux-Init-HOWTO-5.html">Next</A>
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<A HREF="Linux-Init-HOWTO-3.html">Previous</A>
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<A HREF="Linux-Init-HOWTO.html#toc4">Contents</A>
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<HR>
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<H2><A NAME="s4">4. Linux architecture-specific initialization</A></H2>
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<P>
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<P>(from "linux/arch/i386/kernel/head.S")
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<P>The boot code in "linux/arch/i386/boot/setup.S" transfers execution
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to the beginning code in "linux/arch/i386/kernel/head.S"
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(labeled "startup_32:").
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<P>To get to this point, a small uncompressed kernel function
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decompresses the remaining compressed kernel image
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and then it jumps to the new kernel code.
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<P>This is a description of what the "head.S" code does.
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<P>
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<H2><A NAME="ss4.1">4.1 startup_32:</A>
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</H2>
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<P>
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<P>swapper_pg_dir is the top-level page directory, address 0x00101000.
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<P>On entry, %esi points to the real-mode code as a 32-bit pointer.
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<P>
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<H2><A NAME="ss4.2">4.2 Set segment registers to known values</A>
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</H2>
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<P>
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<P>Set the %ds, %es, %fs, and %gs registers to __KERNEL_DS.
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<P>
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<H2><A NAME="ss4.3">4.3 SMP BSP (Bootstrap Processor) check</A>
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</H2>
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<P>
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<P>#ifdef CONFIG_SMP
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<P>If %bx is zero, this is a boot on the Bootstrap Processor (BSP),
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so skip this. Otherwise, for an AP (Application Processor):
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<P>If the desired %cr4 setting is non-zero, turn on the paging options
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(PSE, PAE, ...) and skip "Initialize page tables" (jump to "Enable paging").
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<P>#endif /* CONFIG_SMP */
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<P>
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<H2><A NAME="ss4.4">4.4 Initialize page tables</A>
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</H2>
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<P>
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<P>Begin at pg0 (page 0) and init all pages to 007 (PRESENT + RW + USER).
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<P>
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<H2><A NAME="ss4.5">4.5 Enable paging</A>
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</H2>
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<P>
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<P>Set %cr3 (page table pointer) to swapper_pg_dir.
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<P>Set the paging ("PG") bit of %cr0 to <BR>
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<B>********** enable paging **********</B>.
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<P>Jump $ to flush the prefetch queue.
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<P>Jump *[$] to make sure that %eip is relocated.
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<P>Setup the stack pointer (lss stack_start, %esp).
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<P>#ifdef CONFIG_SMP
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<P>If this is not the BSP (Bootstrap Processor), clear all flags bits
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and jump to checkCPUtype.
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<P>#endif /* CONFIG_SMP */
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<P>
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<H2><A NAME="ss4.6">4.6 Clear BSS</A>
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</H2>
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<P>
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<P>The BSP clears all of BSS (area between __bss_start and _end)
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for the kernel.
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<P>
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<H2><A NAME="ss4.7">4.7 32-bit setup</A>
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</H2>
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<P>
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<P>Setup the IDT for 32-bit mode (call setup_idt).
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setup_idt sets up an IDT with 256 entries pointing to the default
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interrupt handler "ignore_int" as interrupt gates. It doesn't actually
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load the IDT; that can be done only after paging has been enabled
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and the kernel moved to PAGE_OFFSET. Interrupts
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are enabled elsewhere, when we can be relatively
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sure everything is OK.
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<P>Clear the eflags register (before switching to protected mode).
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<P>
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<H2><A NAME="ss4.8">4.8 Copy boot parameters and command line out of the way</A>
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</H2>
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<P>
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<P>First 2 KB of _empty_zero_page is for boot parameters,
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second 2 KB is for the command line.
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<P>
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<H2><A NAME="ss4.9">4.9 checkCPUtype</A>
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</H2>
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<P>
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<P>Initialize X86_CPUID to -1.
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<P>Use Flags register, push/pop results, and CPUID instruction(s) to
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determine CPU type and vendor:
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Sets X86, X86_CPUID, X86_MODEL, X86_MASK, and X86_CAPABILITY.
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Sets bits in %cr0 accordingly.
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<P>Also checks for presence of an 80287 or 80387 coprocessor.
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Sets X86_HARD_MATH if a math coprocessor or floating point unit is found.
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<P>
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<H2><A NAME="ss4.10">4.10 Count this processor</A>
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</H2>
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<P>
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<P>For CONFIG_SMP builds, increment the "ready" counter to keep a tally
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of the number of CPUs that have been initialized.
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<P>
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<H2><A NAME="ss4.11">4.11 Load descriptor table pointer registers</A>
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</H2>
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<P>
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<P>Load GDT with gdt_descr and IDT with idt_descr.
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The GDT contains 2 entries for the kernel (4 GB each for code and
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data, beginning at 0) and 2 userspace entries (4 GB each for code and
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data, beginning at 0). There are 2 null descriptors between the
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userspace descriptors and the APM descriptors.
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<P>The GDT also contains 4 entries for APM segments.
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The APM segments have byte granularity and their bases and limits
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are set at runtime.
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<P>The rest of the gdt_table (after the APM segments) is space for
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TSSes and LDTs.
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<P>Jump to __KERNEL_CS:%eip to cause the GDT to be used. Now in <BR>
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<B>********** protected mode **********</B>.
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<P>Reload all of the segment registers:
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Set the %ds, %es, %fs, and %gs registers to __KERNEL_DS.
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<P>#ifdef CONFIG_SMP
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<P>Reload the stack pointer segment only (%ss) with __KERNEL_DS.
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<P>#else /* not CONFIG_SMP */
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<P>Reload the stack pointer (%ss:%esp) with stack_start.
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<P>#endif /* CONFIG_SMP */
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<P>Clear the LDT pointer to 0.
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<P>Clear the processor's Direction Flag (DF) to 0 for gcc.
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<P>
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<H2><A NAME="ss4.12">4.12 Start other processors</A>
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</H2>
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<P>
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<P>For CONFIG_SMP builds,
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if this is not the first (Bootstrap) CPU, call initialize_secondary(),
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which does not return. The secondary (AP) processor(s) are
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initialized and then enter idle state until processes are
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scheduled on them.
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<P>If this is the first or only CPU, call start_kernel(). (see below)
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<P>/* the calls above should never return, but in case they do: */
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<P>L6: jmp L6
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<P>
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<HR>
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<A HREF="Linux-Init-HOWTO-5.html">Next</A>
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<A HREF="Linux-Init-HOWTO-3.html">Previous</A>
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<A HREF="Linux-Init-HOWTO.html#toc4">Contents</A>
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