.code16
.section .boot,"a"
- mov $0x13, %ax
- int $0x10
-
- mov $1, %al
- mov $0x3c8, %dx
- outb %al, %dx
- mov $0x3c9, %dx
- mov $63, %al
- outb %al, %dx
- xor %al, %al
- outb %al, %dx
- outb %al, %dx
-
- mov $200, %ebx
- mov $0x00000101, %eax
- pushl $0xa000
- popl %es
- xor %di, %di
-fill:
- mov %ebx, %ecx
- and $1, %ecx
- jnz 0f
- rol $16, %eax
-0: mov $80, %ecx
- rep stosl
- dec %ebx
- jnz fill
+ cli
+ cld
+ # move stack to just below the code
+ xor %ax, %ax
+ mov %ax, %ss
+ mov $0x7c00, %sp
+ # use the code segment for data access
+ mov %cs, %ax
+ mov %ax, %ds
+ mov %ax, %es
+
+ mov %dl, drive_number
+
+ call clearscr
+
+ mov $_boot2_size, %eax
+ call print_num
+
+ # load the second stage boot loader and jump to it
+ mov $_boot2_size, %eax
+ mov %eax, %ebx
+ shr $9, %eax
+ and $0x1ff, %ebx
+ jz 0f
+ inc %ax
+0: pushw %ax
+ pushw $1
+ # set es to the start of the destination buffer to allow reading in
+ # full 64k chunks
+ mov $boot2_addr, %bx
+ shr $4, %bx
+ mov %bx, %es
+ xor %bx, %bx
+ call read_sectors
+ jmp boot2_addr
cli
hlt
+ .set SECT_PER_TRACK, 18
+
+ .set ARG_NSECT, 6
+ .set ARG_SIDX, 4
+
+# read_sectors(first, num)
+read_sectors:
+ push %bp
+ mov %sp, %bp
+
+ mov ARG_SIDX(%bp), %ax
+ xor %cx, %cx
+
+ jmp 1f
+0: push %ax
+ call read_sector
+ pop %ax
+ inc %ax
+ inc %cx
+1: cmp ARG_NSECT(%bp), %cx
+ jnz 0b
+
+ pop %bp
+ ret
+
+ .set VAR_ATTEMPTS, -2
+
+# read_sector(sidx)
+read_sector:
+ push %bp
+ mov %sp, %bp
+ sub $2, %sp
+ push %cx
+ push %dx
+
+ movw $3, VAR_ATTEMPTS(%bp)
+
+.Lread_try:
+ # calculate the track (sidx / sectors_per_track)
+ mov 4(%bp), %ax
+ xor %dx, %dx
+ mov $SECT_PER_TRACK, %cx
+ div %cx
+ mov %ax, %cx
+ # save the remainder in ax
+ mov %dx, %ax
+ # head in dh
+ mov %cl, %dh
+ and $1, %dh
+ # cylinder (track/2) in ch [0-7] and cl[6,7]<-[8,9]
+ rol $7, %cx
+ ror $2, %cl
+ and $0xc0, %cl
+ # sector num cl[0-5] is sidx % sectors_per_track (saved in ax)
+ inc %al
+ or %al, %cl
+ # ah = 2 (read), al = 1 sectors
+ mov $0x0201, %ax
+ movb drive_number, %dl
+ int $0x13
+ jnc .Lread_ok
+
+ # abort after 3 attempts
+ decw VAR_ATTEMPTS(%bp)
+ jz .Lread_fail
+
+ # error detected, reset controller and retry
+ xor %ah, %ah
+ int $0x13
+ jmp .Lread_try
+
+.Lread_fail:
+ mov 4(%bp), %ax
+ jmp abort_read
+
+.Lread_ok:
+ # increment es:bx accordingly (advance es if bx overflows)
+ add $512, %bx
+ jno 0f
+ mov %es, %ax
+ add $4096, %ax
+ mov %ax, %es
+
+0: pop %dx
+ pop %cx
+ add $2, %sp
+ pop %bp
+ ret
+
+str_read_error: .asciz "Failed to read sector: "
+
+abort_read:
+ push %ax
+ mov $str_read_error, %si
+ call print_str
+
+ xor %eax, %eax
+ pop %ax
+ call print_num
+
+ cli
+ hlt
+
+clearscr:
+ push %es
+ pushw $0xb800
+ pop %es
+ xor %eax, %eax
+ xor %di, %di
+ movl $500, %ecx
+ rep stosl
+ pop %es
+ ret
+
+cursor_x: .byte 0
+
+# expects string pointer in ds:si
+print_str:
+ push %es
+ pushw $0xb800
+ pop %es
+ xor %di, %di
+ movb $0, cursor_x
+
+0: mov (%si), %al
+ mov %al, %es:(%di)
+ movb $7, %es:1(%di)
+ inc %si
+ add $2, %di
+ incb cursor_x
+ cmp $0, %al
+ jnz 0b
+
+ pop %es
+ ret
+
+# expects number in eax
+print_num:
+ # save es
+ push %es
+
+ xor %cx, %cx
+ movw $numbuf, %si
+ mov $10, %ebx
+
+0: xor %edx, %edx
+ div %ebx
+ add $48, %dl
+ mov %dl, (%si)
+ inc %si
+ inc %cx
+ cmp $0, %eax
+ jnz 0b
+
+ # print the backwards string
+ pushw $0xb800
+ pop %es
+ movb cursor_x, %al
+ xor %ah, %ah
+ shl $1, %ax
+ mov %ax, %di
+
+0: dec %si
+ mov (%si), %al
+ movb %al, %es:(%di)
+ inc %di
+ mov $7, %al
+ movb %al, %es:(%di)
+ inc %di
+ dec %cx
+ jnz 0b
+
+ # restore es
+ pop %es
+ ret
+
+drive_number: .byte 0
+numbuf: .space 10
.org 510
.byte 0x55
.byte 0xaa
+
+boot2_addr:
projects / bootcensus / blobdiff