368 lines
7.4 KiB
ArmAsm
368 lines
7.4 KiB
ArmAsm
// SPDX-License-Identifier: GPL-2.0
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//
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// bootsect.S supports booting directly from the BIOS or via an intermediate
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// bootloader that supports the Linux boot protocol. If booted directly from
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// the BIOS, it is loaded at address 0x7c00. It then loads setup.S immediately
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// after itself (address 0x7e00) and the main program code at segment MAIN_SEG,
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// using BIOS interrupts to read the data from disk. When using an intermediate
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// bootloader, it provides the first few bytes of the Linux boot header (at the
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// end of the boot sector), with the remainder of the header being provided by
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// setup.S.
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//
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// Copyright (C) 2020 Martin Whitaker.
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//
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// Derived from memtest86+ bootsect.S:
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//
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// bootsect.s Copyright (C) 1991, 1992 Linus Torvalds
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//
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// 1-Jan-96 Modified by Chris Brady for use as a boot loader for MemTest-86.
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#define __ASSEMBLY__
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#include "boot.h"
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.section ".bootsect", "ax", @progbits
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.code16
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# The BIOS boot entry point. This will be located at 0x7c00.
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.globl boot
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boot:
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# Initialise the segment registers and the stack.
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ljmp $BOOT_SEG, $init
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init:
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movw %cs, %ax
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movw %ax, %ds
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movw %ax, %es
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movw %ax, %ss
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movw $BOOT_STACK_TOP, %ax
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movw %ax, %sp
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# Many BIOS's default disk parameter tables will not recognize
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# multi-sector reads beyond the maximum sector number specified
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# in the default diskette parameter tables - this may mean 7
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# sectors in some cases.
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#
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# Since single sector reads are slow and out of the question,
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# we must take care of this by creating new parameter tables
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# (for the first disk) in RAM. We will set the maximum sector
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# count to 18 - the most we will encounter on an HD 1.44.
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#
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# High doesn't hurt. Low does.
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#
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# Segments are as follows:
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# ds=es=ss=cs = BOOT_SEG,
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# fs = 0, gs = parameter table segment
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pushw $0
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popw %fs
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movw $0x78, %bx # fs:bx is parameter table address
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lgs %fs:(%bx),%si # gs:si is source
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movw %dx, %di # es:di is destination
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movw $6, %cx # copy 12 bytes
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cld
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rep movsw %gs:(%si), (%di)
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movw %dx, %di
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movb $18, 4(%di) # patch sector count
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movw %di, %fs:(%bx)
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movw %es, %fs:2(%bx)
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movw %cs, %ax
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movw %ax, %fs
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movw %ax, %gs
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xorb %ah, %ah # reset FDC
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xorb %dl, %dl
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int $0x13
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# Load the setup sectors directly after the boot block.
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# Note that 'es' is already set up.
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load_setup:
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xorw %dx, %dx # drive 0, head 0
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movw $0x0002, %cx # sector 2, track 0
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movw $0x0200, %bx # address = 512, in BOOT_SEG
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movw $(0x0200 + SETUP_SECS), %ax # service 2, nr of sectors
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# (assume all on head 0, track 0)
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int $0x13 # read it
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jnc load_setup_done # ok - continue
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pushw %ax # dump error code
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call print_nl
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movw %sp, %bp
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call print_hex
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popw %ax
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xorb %dl, %dl # reset FDC
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xorb %ah, %ah
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int $0x13
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jmp load_setup
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load_setup_done:
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# Get disk drive parameters, specifically number of sectors/track.
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# It seems that there is no BIOS call to get the number of sectors.
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# Guess 18 sectors if sector 18 can be read, 15 if sector 15 can be
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# read. Otherwise guess 9.
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xorw %dx, %dx # drive 0, head 0
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movw $0x0012, %cx # sector 18, track 0
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movw $BOOT_STACK, %bx # use the bottom of the stack (es = cs)
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movw $0x0201, %ax # service 2, 1 sector
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int $0x13
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jnc got_sectors
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movb $0x0f, %cl # sector 15
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movw $0x0201, %ax # service 2, 1 sector
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int $0x13
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jnc got_sectors
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movb $0x09, %cl
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got_sectors:
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movw %cx, %cs:sectors
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movw $BOOT_SEG, %ax
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movw %ax, %es
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# Print a message.
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movb $0x03, %ah # read cursor pos
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xorb %bh, %bh
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int $0x10
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leaw boot_msg, %bp
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movw $(boot_msg_end - boot_msg), %cx
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movw $0x0007, %bx # page 0, attribute 7 (normal)
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movw $0x1301, %ax # write string, move cursor
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int $0x10
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# Load the main test program.
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movw $MAIN_SEG, %ax
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movw %ax, %es
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call read_it
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call kill_motor
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call turn_off_cursor
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call print_nl
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# Fix up the Linux boot header to indicate we've loaded into low memory.
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movl $LOW_LOAD_ADDR, code32_start
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# After that (everything loaded), we jump to the setup code loaded
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# directly after the boot block.
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ljmp $SETUP_SEG, $0
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# This subroutine loads the system at address 0x10000, making sure no 64KB
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# boundaries are crossed. We try to load it as fast as possible, loading
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# whole tracks whenever we can.
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#
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# in: es - starting address segment (normally 0x1000)
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#
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sread: .word 1 + SETUP_SECS # sectors read of current track
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head: .word 0 # current head
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track: .word 0 # current track
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read_it:
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movw %es, %ax
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testw $0x0fff, %ax
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die:
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jne die # es must be at 64kB boundary
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xorw %bx,%bx # bx is starting address within segment
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rp_read:
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movw %es, %ax
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subw $MAIN_SEG, %ax # have we loaded all yet?
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cmpw sys_size, %ax
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jbe ok1_read
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ret
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ok1_read:
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movw %cs:sectors, %ax
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subw sread, %ax
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movw %ax, %cx
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shlw $9, %cx
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addw %bx, %cx
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jnc ok2_read
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je ok2_read
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xorw %ax, %ax
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subw %bx, %ax
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shrw $9, %ax
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ok2_read:
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call read_track
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movw %ax, %cx
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add sread, %ax
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cmpw %cs:sectors, %ax
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jne ok3_read
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movw $1, %ax
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subw head, %ax
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jne ok4_read
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incw track
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ok4_read:
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movw %ax, head
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xorw %ax, %ax
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ok3_read:
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movw %ax, sread
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shlw $9, %cx
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addw %cx, %bx
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jnc rp_read
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movw %es, %ax
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addb $0x10, %ah
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movw %ax, %es
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xorw %bx, %bx
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jmp rp_read
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read_track:
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pusha
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pusha
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movw $0xe2e, %ax # loading... message 2e = .
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movw $7, %bx
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int $0x10
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popa
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movw track, %dx
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movw sread, %cx
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incw %cx
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movb %dl, %ch
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movw head, %dx
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movb %dl, %dh
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andw $0x0100, %dx
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movb $2, %ah
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pushw %dx # save for error dump
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pushw %cx
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pushw %bx
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pushw %ax
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int $0x13
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jc bad_rt
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addw $8, %sp
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popa
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ret
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bad_rt:
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pushw %ax # save error code
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call print_all # ah = error, al = read
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xorb %ah, %ah
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xorb %dl, %dl
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int $0x13
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addw $10, %sp
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popa
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jmp read_track
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# This subroutine is for debugging purposes. It will print out all of the
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# registers. The assumption is that this is called from a routine, with a
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# stack frame like:
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# dx
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# cx
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# bx
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# ax
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# err
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# ret <- sp
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print_all:
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movw $5, %cx # error code + 4 registers
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movw %sp, %bp
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print_loop:
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pushw %cx # save count left
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call print_nl # nl for readability
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cmpb 5, %cl # see if register name is needed
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jae no_reg
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movw $(0xe05 + 'A' - 1), %ax
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subb %cl, %al
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int $0x10
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movb $'X', %al
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int $0x10
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movb $':', %al
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int $0x10
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no_reg:
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addw $2, %bp # next register
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call print_hex # print it
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popw %cx
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loop print_loop
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ret
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print_nl:
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movw $0xe0d, %ax # CR
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int $0x10
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movb $0x0a, %al # LF
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int $0x10
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ret
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# This subroutine is for debugging purposes, and prints the word pointed to
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# by ss:bp in hexadecimal.
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print_hex:
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movw $4, %cx # 4 hex digits
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movw (%bp), %dx # load word into dx
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print_digit:
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rolw $4, %dx # rotate so that lowest 4 bits are used
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movb $0xe, %ah
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movb %dl, %al # mask off so we have only next nibble
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andb $0xf, %al
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addb $'0', %al # convert to 0-based digit
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cmpb $'9', %al # check for overflow
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jbe good_digit
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addb $('A' - '0' - 10), %al
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good_digit:
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int $0x10
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loop print_digit
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ret
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# This subroutine turns off the floppy drive motor, so that we enter the
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# kernel in a known state, and don't have to worry about it later.
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kill_motor:
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pushw %dx
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movw $0x3f2, %dx
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xorb %al, %al
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outb %al, %dx
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popw %dx
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ret
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# This subroutine turns off the text display cursor.
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turn_off_cursor:
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movb $0x01, %ah
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movb $0x00, %bh
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movw $0x2000, %cx
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int $0x10
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ret
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# Local variables.
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sectors:
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.word 0
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boot_msg:
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.ascii "Loading Memtest86+"
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boot_msg_end:
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# Emulate the Linux boot header, to allow loading by intermediate boot loaders.
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.org 497
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setup_sects:
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.byte SETUP_SECS
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root_flags:
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.word 0
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sys_size:
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.long _sys_size
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ram_size:
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.word 0
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vid_mode:
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.word 0
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root_dev:
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.word 0
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boot_flag:
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.word 0xAA55
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