65 lines
1.5 KiB
ArmAsm
65 lines
1.5 KiB
ArmAsm
/* https://github.com/cirosantilli/x86-bare-metal-examples#protected-mode-segmentation */
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#include "common.h"
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BEGIN
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CLEAR
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STAGE2
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PROTECTED_MODE
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/* Sanity check 1: just print the output. */
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VGA_PRINT_STRING $output
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/* Sanity check 2: make a mov without any segment manipulation. */
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mov message, %cl
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mov %cl, output
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VGA_PRINT_STRING $output
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/* Now for the real action. */
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mov $gdt_data, %edx
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/* We are touching the 7th byte of the data entry. */
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add $3, %edx
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mov (%edx), %al
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/* Cache it for later. */
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mov %al, %bl
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/* Set the first bit of the descriptor memory. */
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xor $1, %al
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mov %al, (%edx)
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/* We must re-set ds because the segment descriptor is cached
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* and this updates it:
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* http://wiki.osdev.org/Descriptor_Cache
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*/
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mov $DATA_SEG, %ax
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mov %ax, %ds
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/* This is the only memory access we will make with
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* the modified segment, to minimize the effect on our IO.
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*/
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mov message, %cl
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/* Restore the old segment. */
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/* TODO is this needed to take into account the new segmentation? */
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dec %edx
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mov %bl, (%edx)
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mov %ax, %ds
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/* TODO this sanity check is not printing "ab".
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* It fails, so we're not restoring the old state properly.
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* Maybe blows up because video memory going wrong?
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*/
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VGA_PRINT_STRING $message
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mov %cl, output
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VGA_PRINT_STRING $output
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hlt
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message:
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.asciz "ab"
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output:
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.asciz "x"
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