diff options
Diffstat (limited to 'src/boot_asm.S')
| -rw-r--r-- | src/boot_asm.S | 91 |
1 files changed, 33 insertions, 58 deletions
diff --git a/src/boot_asm.S b/src/boot_asm.S index 018f096..17823d6 100644 --- a/src/boot_asm.S +++ b/src/boot_asm.S @@ -1,5 +1,5 @@ /* - * Copyright (c) 2017 Richard Braun. + * Copyright (c) 2017-2018 Richard Braun. * Copyright (c) 2017 Jerko Lenstra. * * Permission is hereby granted, free of charge, to any person obtaining a @@ -21,47 +21,9 @@ * DEALINGS IN THE SOFTWARE. */ +#include "asm.h" #include "boot.h" - -/* - * These are values used in the OS image header, as defined by the multiboot - * specification. - * - * See https://www.gnu.org/software/grub/manual/multiboot/multiboot.html. - */ -#define BOOT_HDR_MAGIC 0x1BADB002 -#define BOOT_HDR_CHECK 0x2BADB002 -#define BOOT_HDR_FLAGS 0x0 - -/* - * The .section directive tells the assembler which section the following - * instructions should go into. - * - * The "a" flag makes the section allocatable, meaning memory will be - * allocated for that section at load time. - * - * See https://sourceware.org/binutils/docs-2.29/as/Section.html#Section. - */ -.section .hdr, "a" - -/* Generate code for i386 */ -.code32 - -/* - * The .int directive is used to emit verbatim machine words. Here, the - * third word is the checksum of the first two, defined as "a 32-bit - * unsigned value which, when added to the other magic fields (i.e. - * ‘magic’ and ‘flags’), must have a 32-bit unsigned sum of zero". - * Intuitively, adding the two first words and making the result negative - * gives a value that, when added to the other fields, gives 0, despite - * the word being unsigned. This trick works because values use two's - * complement representation. - * - * See https://en.wikipedia.org/wiki/Two%27s_complement. - */ -.int BOOT_HDR_MAGIC -.int BOOT_HDR_FLAGS -.int -(BOOT_HDR_FLAGS + BOOT_HDR_MAGIC) +#include "cpu.h" /* * Put the following instructions into the .text section, which is @@ -74,23 +36,36 @@ * be run when control is passed to the kernel. The address of this symbol * is what the following command returns : * readelf -aW x1 | grep "Entry point" + */ +ASM_FUNC(boot_start) + mov %r0, %r13 /* load boot stack in PSP */ + msr psp, %r0 + mov %r0, $2 /* use PSP as stack pointer */ + msr control, %r0 + isb /* force CONTROL register evaluation, + i.e. R13 is now PSP */ + ldr %r0, =cpu_exc_stack /* load exception stack in MSP */ + add %r0, %r0, CPU_EXC_STACK_SIZE + msr msp, %r0 + bl boot_clear_bss /* boot_clear_bss() */ + b boot_main /* goto boot_main */ + +/* + * Clear the .bss section. * - * The .global directive tells the assembler to make the symbol global, - * i.e. to make it visible to other compilation units. - * - * When this code is run, the machine state should comply with what the - * multiboot specification defines. + * The stack, being part of the .bss section, must not be used to save + * data, since that data will be cleared. */ -.global boot_start -boot_start: - cmp $BOOT_HDR_CHECK, %eax /* Compare EAX against the expected value */ - jne . /* If not equal, jump to the current address. - This is an infinite loop. */ - mov $boot_stack, %esp /* Set up a stack */ - add $BOOT_STACK_SIZE, %esp /* On x86, stacks grow downwards, so start - at the top */ - jmp main /* Jump to the C main function */ +ASM_FUNC(boot_clear_bss) /* void */ + mov %r0, #0 /* boot_clear_bss(void) */ + ldr %r1, =_bss_start /* { */ + ldr %r2, =_bss_end /* uint32_t *r1 = &_bss_start; */ + /* uint32_t *r2 = &_bss_end; */ -loop: - hlt /* Never reached, for safety */ - jmp loop +1: + cmp %r1, %r2 /* while (r1 < r2) { */ + it hs /* *r1 = 0; */ + bxhs %r14 /* r1++; */ + str %r0, [%r1] /* } */ + add %r1, %r1, #4 /* } */ + b 1b |