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Diffstat (limited to 'src/arch/x86_64.md')
| -rw-r--r-- | src/arch/x86_64.md | 64 |
1 files changed, 63 insertions, 1 deletions
diff --git a/src/arch/x86_64.md b/src/arch/x86_64.md index 1f7ffdd..8bcc308 100644 --- a/src/arch/x86_64.md +++ b/src/arch/x86_64.md @@ -312,7 +312,7 @@ must must save these registers in case they are used. - `r12` - `r15` - `xmm6` - `xmm15` -## ASM skeleton +## ASM skeleton - linux userspace Small assembler skeleton, ready to use with following properties: - use raw Linux syscalls (`man 2 syscall` for ABI) - no `C runtime (crt)` @@ -350,6 +350,67 @@ To compile and run: Hi ASM-World! ``` +## MBR boot sectors example +The following shows a non-minimal [MBR][mbr] boot sector, which transitions from +16-bit _real mode_ to 32-bit _protected mode_ by setting up a small _global +descriptor table (GDT)_. A string is printed in each mode. + +```x86asm +{{ #include x86/mbr/mbr.S }} +``` + +The linker script. +```x86asm +{{ #include x86/mbr/mbr.ld }} +``` + +The build instructions. +```make +{{ #include x86/mbr/Makefile:1:6 }} +``` + +One can boot into the bootsector from legacy BIOS, either with qemu or by +writing the mbr boot sector as first sector onto a usb stick. + +```sh +qemu-system-i386 -hda mbr +``` + +The following gives some more detailed description for the _segment selector_ +registers, the _segment descriptors_ in the GDT, and the _GDT descriptor_ +itself. +``` +# Segment Selector (cs, ds, es, ss, fs, gs). + +[15:3] I Descriptor Index + [2:1] TI Table Indicator (0=GTD | 1=LDT) + [0] RPL Requested Privilege Level + + +# Segment Descriptor (2 x 4 byte words). + +0x4 [31:24] Base[31:24] +0x4 [23] G Granularity, scaling of limit (0=1B | 1=4K) +0x4 [22] D/B (0=16bit | 1=32bit) +0x4 [21] L (0=compatibility mode | 1=64bit code) if 1 -> D/B = 0 +0x4 [20] AVL Free use for system sw +0x4 [19:16] Limit[19:16] +0x4 [15] P Present +0x4 [14:13] DPL Descriptor privilege level +0x4 [12] S (0=system segment | 1=code/data) +0x4 [11:0] Type Code or data and access information. +0x4 [7:0] Base[23:16] + +0x0 [31:16] Base[15:0] +0x0 [15:0] Limit[15:0] + + +# GDT descriptor (32bit mode) + +[47:16] Base address of GDT table. + [15:0] Length of GDT table. +``` + ## References - [SystemV AMD64 ABI][sysvabi] - [AMD64 Vol1: Application Programming][amd64_vol1] @@ -379,3 +440,4 @@ Hi ASM-World! [gas_directives]: https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops [gas_x86_64]: https://sourceware.org/binutils/docs/as/i386_002dDependent.html [juicebox]: https://github.com/johannst/juicebox-asm +[mbr]: https://en.wikipedia.org/wiki/Master_boot_record |