+# pcboot - bootable PC demo/game kernel
+# Copyright (C) 2018-2019 John Tsiombikas <nuclear@member.fsf.org>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY, without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <https://www.gnu.org/licenses/>.
+
.code16
- .section .boot,"a"
+ .section .boot,"ax"
+
+ .set stack_top, 0x7be0
+ .set read_retries, 0x7be8
+ .set drive_number, 0x7bec
+ .set cursor_x, 0x7bee
+ .set scratchbuf, 0x7bf0
+ .set scratchbuf_size, 16
+boot:
cli
- cld
# move stack to just below the code
xor %ax, %ax
mov %ax, %ss
- mov $0x7c00, %sp
+ mov $stack_top, %sp
# use the code segment for data access
mov %cs, %ax
mov %ax, %ds
mov %dl, drive_number
- call clearscr
+ call get_drive_chs
+ mov $loading_msg, %si
+ call print_str
+
+ # load the second stage boot loader and jump to it
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
call read_sectors
jmp boot2_addr
- cli
- hlt
+loading_msg: .asciz "\nLoad "
+driveno_msg: .asciz "Drv:"
+
+ .global sect_per_track
+sect_per_track: .short 18
+ .global num_cylinders
+num_cylinders: .short 80
+ .global num_heads
+num_heads: .short 2
+ .global heads_mask
+heads_mask: .byte 1
+
+get_drive_chs:
+ mov $driveno_msg, %si
+ call print_str
+ xor %eax, %eax
+ movb drive_number, %dl
+ mov %dl, %al
+ call print_num
+ mov $10, %al
+ call print_char
+
+ mov $8, %ah
+ int $0x13
+ jnc ok
+ ret
+
+ok: xor %eax, %eax
+ mov %ch, %al
+ mov %cl, %ah
+ rol $2, %ah
+ inc %ax
+ and $0x3ff, %ax
+ mov %ax, num_cylinders
+
+ and $0x3f, %cx
+ mov %cx, sect_per_track
+
+ shr $8, %dx
+ mov %dl, heads_mask
+ inc %dx
+ mov %dx, num_heads
+
+ call print_num
+ mov $47, %al
+ call print_char
+ mov %dx, %ax
+ call print_num
+ mov $47, %al
+ call print_char
+ mov %cx, %ax
+ call print_num
+ ret
- .set SECT_PER_TRACK, 18
.set ARG_NSECT, 6
.set ARG_SIDX, 4
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)
+ movw $3, read_retries
-.Lread_try:
+read_try:
# calculate the track (sidx / sectors_per_track)
mov 4(%bp), %ax
+
xor %dx, %dx
- mov $SECT_PER_TRACK, %cx
+ mov sect_per_track, %cx
div %cx
mov %ax, %cx
- # save the remainder in ax
- mov %dx, %ax
+ # save the remainder
+ push %dx
# 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
+ and heads_mask, %dh
+ # cylinder (track/heads) in ch [0-7] and cl[6,7]<-[8,9]
+ push %dx
+ xor %dx, %dx
+ movw num_heads, %cx
+ div %cx
+ pop %dx
+ mov %ax, %cx
+ rol $8, %cx
ror $2, %cl
and $0xc0, %cl
- # sector num cl[0-5] is sidx % sectors_per_track (saved in ax)
+ # sector num cl[0-5] is sidx % sectors_per_track + 1
+ pop %ax
inc %al
- or %al, %cl
+ or %al, %cl
+
# ah = 2 (read), al = 1 sectors
mov $0x0201, %ax
movb drive_number, %dl
int $0x13
- jnc .Lread_ok
+ jnc read_ok
# abort after 3 attempts
- decw VAR_ATTEMPTS(%bp)
- jz .Lread_fail
+ decw read_retries
+ jz read_fail
# error detected, reset controller and retry
xor %ah, %ah
int $0x13
- jmp .Lread_try
+ jmp read_try
-.Lread_fail:
+read_fail:
mov 4(%bp), %ax
jmp abort_read
-.Lread_ok:
+read_ok:
+ mov $46, %ax
+ call print_char
+
# increment es:bx accordingly (advance es if bx overflows)
add $512, %bx
- jno 0f
+ jnc 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: "
+str_read_error: .asciz "rderr:"
abort_read:
- push %ax
mov $str_read_error, %si
call print_str
-
- xor %eax, %eax
- pop %ax
+ and $0xffff, %eax
call print_num
-
- cli
+ mov $10, %al
+ call print_char
hlt
-clearscr:
- push %es
- pushw $0xb800
- pop %es
- xor %eax, %eax
- xor %di, %di
- movl $500, %ecx
- rep stosl
- pop %es
- ret
+ # expects string pointer in ds:si
+print_str:
+ pusha
-cursor_x: .byte 0
+0: mov (%si), %al
+ cmp $0, %al
+ jz end
+ call print_char
+ inc %si
+ jmp 0b
-# expects string pointer in ds:si
-print_str:
+end: popa
+ ret
+
+ # expects character in al
+print_char:
push %es
- pushw $0xb800
+
+ cmp $10, %ax
+ jnz 0f
+ mov $32, %ax
+
+0: pushw $0xb800
pop %es
- xor %di, %di
- movb $0, cursor_x
+ movw cursor_x, %di
+ shl $1, %di
-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
+ incw cursor_x
pop %es
ret
-# expects number in eax
+ # expects number in eax
+ .global print_num
print_num:
- # save es
- push %es
+ # save registers
+ pusha
- xor %cx, %cx
- movw $numbuf, %si
+ movw $scratchbuf + scratchbuf_size, %si
+ movb $0, (%si)
mov $10, %ebx
-
-0: xor %edx, %edx
+convloop:
+ xor %edx, %edx
div %ebx
add $48, %dl
+ dec %si
mov %dl, (%si)
- inc %si
- inc %cx
cmp $0, %eax
- jnz 0b
+ jnz convloop
- # 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
+ call print_str
- # restore es
- pop %es
+ # restore regs
+ popa
ret
-drive_number: .byte 0
-numbuf: .space 10
.org 510
.byte 0x55
.byte 0xaa
-
boot2_addr:
projects / bootcensus / blobdiff