-# pcboot - bootable PC demo/game kernel
+# 256boss - bootable launcher for 256byte intros
# Copyright (C) 2018 John Tsiombikas <nuclear@member.fsf.org>
#
# This program is free software: you can redistribute it and/or modify
# along with this program. If not, see <https://www.gnu.org/licenses/>.
# this is the second-stage boot loader
+# plus some other code that needs to run below 1mb (int86 implementation).
+
.code16
.section .boot2,"ax"
.set main_load_addr, 0x100000
+ .set drive_number, 0x7bec
# make sure any BIOS call didn't re-enable interrupts
cli
+ xor %eax, %eax
+ mov drive_number, %al
+ mov %eax, boot_drive_number
+
+ call setup_serial
+
# enter unreal mode
call unreal
# enable A20 address line
call enable_a20
+ # detect available memory
+ call detect_memory
+
# load the whole program into memory starting at 1MB
call load_main
- #mov $0x13, %ax
- #int $0x10
-
# load initial GDT
lgdt (gdt_lim)
# load initial IDT
# the BIOS might have enabled interrupts
cli
- # just in case we were loaded from floppy, turn all floppy motors off
+ # if we were loaded from floppy, turn all floppy motors off
+ movb drive_number, %bl
+ and $0x80, %bl
+ jnz 0f
mov $0x3f2, %dx
in %dx, %al
- and $0xf0, %al
+ and $0xf, %al
out %al, %dx
+0:
mov $10, %ax
call putchar
read_retries: .short 0
- .set drive_number, 0x7bec
read_track:
# set es to the start of the destination buffer to allow reading in
# full 64k chunks if necessary
clearscr:
mov $0xb8000, %edi
- xor %eax, %eax
+ # clear with white-on-black spaces
+ mov $0x07200720, %eax
mov $1000, %ecx
addr32 rep stosl
ret
.set UART_DATA, 0x3f8
+ .set UART_DIVLO, 0x3f8
+ .set UART_DIVHI, 0x3f9
+ .set UART_FIFO, 0x3fa
+ .set UART_LCTL, 0x3fb
+ .set UART_MCTL, 0x3fc
.set UART_LSTAT, 0x3fd
+ .set DIV_9600, 115200 / 9600
+ .set LCTL_8N1, 0x03
+ .set LCTL_DLAB, 0x80
+ .set FIFO_ENABLE_CLEAR, 0x07
+ .set MCTL_DTR_RTS_OUT2, 0x0b
.set LST_TREG_EMPTY, 0x20
+setup_serial:
+ # set clock divisor
+ mov $LCTL_DLAB, %al
+ mov $UART_LCTL, %dx
+ out %al, %dx
+ mov $DIV_9600, %ax
+ mov $UART_DIVLO, %dx
+ out %al, %dx
+ shr $8, %ax
+ mov $UART_DIVHI, %dx
+ out %al, %dx
+ # set format 8n1
+ mov $LCTL_8N1, %al
+ mov $UART_LCTL, %dx
+ out %al, %dx
+ # clear and enable fifo
+ mov $FIFO_ENABLE_CLEAR, %al
+ mov $UART_FIFO, %dx
+ out %al, %dx
+ # assert RTS and DTR
+ mov $MCTL_DTR_RTS_OUT2, %al
+ mov $UART_MCTL, %dx
+ out %al, %dx
+ ret
+
+
ser_putchar:
push %dx
numbuf: .space 16
-# this is not boot loader code. It's called later on by the main kernel
-# code in 32bit protected mode. It's placed here because it needs to be
-# located in base memory as it returns and runs in real mode.
- .code32
- .align 4
- # place to save the protected mode IDTR pseudo-descriptor
- # with sidt, so that it can be restored before returning
- .short 0
-saved_idtr:
-idtlim: .short 0
-idtaddr:.long 0
- # real mode IDTR pseudo-descriptor pointing to the IVT at addr 0
- .short 0
-rmidt: .short 0x3ff
- .long 0
+detect_memory:
+ mov $memdet_e820_msg, %esi
+ call putstr
+ call detect_mem_e820
+ jnc memdet_done
+ mov $rdfail_msg, %esi
+ call putstr
-saved_esp: .long 0
-saved_ebp: .long 0
+ mov $memdet_e801_msg, %esi
+ call putstr
+ call detect_mem_e801
+ jnc memdet_done
+ mov $rdfail_msg, %esi
+ call putstr
- # drop back to unreal mode to call 16bit interrupt
- .global int86
-int86:
- push %ebp
- mov %esp, %ebp
- pushal
- cli
- # save protected mode IDTR and replace it with the real mode vectors
- sidt (saved_idtr)
- lidt (rmidt)
-
- # modify the int instruction do this here before the
- # cs-load jumps, to let them flush the instruction cache
- mov $int_op, %ebx
- movb 8(%ebp), %al
- movb %al, 1(%ebx)
-
- # long jump to load code selector for 16bit code (6)
- ljmp $0x30,$0f
-0:
- .code16
- # disable protection
- mov %cr0, %eax
- and $0xfffe, %ax
- mov %eax, %cr0
- # load cs <- 0
- ljmp $0,$0f
-0: # zero data segments
- xor %ax, %ax
- mov %ax, %ds
- mov %ax, %es
- mov %ax, %ss
- nop
+ mov $memdet_88_msg, %esi
+ call putstr
+ call detect_mem_88
+ jnc memdet_done
+ mov $rdfail_msg, %esi
+ call putstr
- # load registers from the int86regs struct
- mov %esp, saved_esp
- mov %ebp, saved_ebp
- mov 12(%ebp), %esp
- popal
- mov saved_esp, %esp
+ # just panic...
+ mov $memdet_fail_msg, %esi
+ call putstr
+0: hlt
+ jmp 0b
- # call 16bit interrupt
-int_op: int $0
+memdet_done:
+ mov $rdok_msg, %esi
+ call putstr
+ ret
- mov saved_ebp, %ebp
- mov 12(%ebp), %esp
- add $34, %esp
- pushfw
- pushal
- mov saved_esp, %esp
+memdet_fail_msg: .ascii "Failed to detect available memory!\n"
+ .ascii "Please file a bug report: https://github.com/jtsiomb/pcboot/issues\n"
+ .asciz " or contact me through email: nuclear@member.fsf.org\n"
+memdet_e820_msg: .asciz "Detecting RAM (BIOS 15h/0xe820)... "
+memdet_e801_msg: .asciz "Detecting RAM (BIOS 15h/0xe801)... "
+memdet_88_msg: .asciz "Detecting RAM (BIOS 15h/0x88, max 64mb)... "
- # re-enable protection
- mov %cr0, %eax
- or $1, %ax
- mov %eax, %cr0
- # long jump to load code selector for 32bit code (1)
- ljmp $0x8,$0f
-0:
- .code32
- # set data selector (2) to all segment regs
- mov $0x10, %ax
- mov %ax, %ds
- mov %ax, %es
- mov %ax, %ss
- nop
+ # detect extended memory using BIOS call 15h/e820
+detect_mem_e820:
+ movl $0, boot_mem_map_size
- # restore 32bit interrupt descriptor table
- lidt (saved_idtr)
- sti
- popal
- pop %ebp
+ mov $buffer, %edi
+ xor %ebx, %ebx
+ mov $0x534d4150, %edx
+
+e820_looptop:
+ mov $0xe820, %eax
+ mov $24, %ecx
+ int $0x15
+ jc e820_fail
+ cmp $0x534d4150, %eax
+ jnz e820_fail
+
+ # skip areas starting above 4GB as we won't be able to use them
+ cmpl $0, 4(%edi)
+ jnz e820_skip
+
+ # only care for type 1 (usable ram), otherwise ignore
+ cmpl $1, 16(%edi)
+ jnz e820_skip
+
+ mov buffer, %eax
+ mov $boot_mem_map, %esi
+ mov boot_mem_map_size, %ebp
+ # again, that's [ebp * 8 + esi]
+ mov %eax, (%esi,%ebp,8)
+
+ # skip areas with 0 size (also clamp size to 4gb)
+ # test high 32bits
+ cmpl $0, 12(%edi)
+ jz e820_highzero
+ # high part is non-zero, make low part ffffffff
+ xor %eax, %eax
+ not %eax
+ jmp 0f
+
+e820_highzero:
+ # if both high and low parts are zero, ignore
+ mov 8(%edi), %eax
+ cmpl $0, %eax
+ jz e820_skip
+
+0: mov %eax, 4(%esi,%ebp,8)
+ incl boot_mem_map_size
+
+e820_skip:
+ # terminate the loop if ebx was reset to 0
+ cmp $0, %ebx
+ jz e820_done
+ jmp e820_looptop
+
+e820_done:
+ clc
ret
+e820_fail:
+ # if size > 0, then it's not a failure, just the end
+ cmpl $0, boot_mem_map_size
+ jnz e820_done
+
+ stc
+ ret
+
+
+ # detect extended memory using BIOS call 15h/e801
+detect_mem_e801:
+ mov $boot_mem_map, %esi
+ mov boot_mem_map_size, %ebp
+ movl $0, (%ebp)
+
+ xor %cx, %cx
+ xor %dx, %dx
+ mov $0xe801, %ax
+ int $0x15
+ jc e801_fail
+
+ cmp $0, %cx
+ jnz 0f
+ cmp $0, %ax
+ jz e801_fail
+ mov %ax, %cx
+ mov %bx, %dx
+
+0: movl $0x100000, (%esi)
+ movzx %cx, %eax
+ # first size is in KB, convert to bytes
+ shl $10, %eax
+ jnc 0f
+ # overflow means it's >4GB, clamp to 4GB
+ mov $0xffffffff, %eax
+0: mov %eax, 4(%esi)
+ incl boot_mem_map_size
+ cmp $0, %dx
+ jz e801_done
+ movl $0x1000000, 8(%esi)
+ movzx %dx, %eax
+ # second size is in 64kb blocks, convert to bytes
+ shl $16, %eax
+ jnc 0f
+ # overflow means it's >4GB, clamp to 4GB
+ mov $0xffffffff, %eax
+0: mov %eax, 12(%esi)
+ incl boot_mem_map_size
+e801_done:
+ clc
+ ret
+e801_fail:
+ stc
+ ret
+
+detect_mem_88:
+ # reportedly some BIOS implementations fail to clear CF on success
+ clc
+ mov $0x88, %ah
+ int $0x15
+ jc x88_fail
+
+ cmp $0, %ax
+ jz x88_fail
+
+ # ax has size in KB, convert to bytes in eax
+ and $0xffff, %eax
+ shl $10, %eax
+
+ mov $boot_mem_map, %esi
+ movl $0x100000, (%esi)
+ mov %eax, 4(%esi)
+
+ movl $1, boot_mem_map_size
+ clc
+ ret
+
+x88_fail:
+ stc
+ ret
+
+
+ # this part is placed at the very end of all low memory sections
+ .section .bootend,"ax"
+ .global boot_mem_map_size
+boot_mem_map_size: .long 0
+ .global boot_mem_map
+boot_mem_map: .space 128
+
+ .align 4
+ .global boot_drive_number
+boot_drive_number:
+ .long 0
# buffer used by the track loader ... to load tracks.
.align 16
buffer:
+ .global low_mem_buffer
+low_mem_buffer:
projects / bootcensus / blobdiff