- memory detection with BIOS 0x15/0xe820

[bootcensus] / src / boot / boot2.s

diff --git a/src/boot/boot2.s b/src/boot/boot2.s
index d44b634..15b1cb1 100644 (file)
--- a/src/boot/boot2.s
+++ b/src/boot/boot2.s
@@ -34,6 +34,9 @@
        # 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
 
@@ -554,6 +557,76 @@ kbc_wait_write:
 numbuf: .space 16
 
 
+       # sets the carry flag on failure
+detect_memory:
+       mov $buffer, %edi
+       xor %ebx, %ebx
+       mov $0x534d4150, %edx
+
+memdet_looptop:
+       mov $0xe820, %eax
+       mov $24, %ecx
+       int $0x15
+       jc memdet_fail
+       cmp $0x534d4150, %eax
+       jnz memdet_fail
+
+       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)
+
+       # only care for type 1 (usable ram), otherwise ignore
+       cmpl $1, 16(%edi)
+       jnz memdet_skip
+
+       # skip areas with 0 size (also clamp size to 4gb)
+       # test high 32bits
+       cmpl $0, 12(%edi)
+       jz memdet_highzero
+       # high part is non-zero, make low part ffffffff
+       xor %eax, %eax
+       not %eax
+       jmp 0f
+
+memdet_highzero:
+       # if both high and low parts are zero, ignore
+       mov 8(%edi), %eax
+       cmpl $0, %eax
+       jz memdet_skip
+
+0:     mov %eax, 4(%esi,%ebp,8)
+       incl boot_mem_map_size
+
+memdet_skip:
+       # terminate the loop if ebx was reset to 0
+       cmp $0, %ebx
+       jz memdet_done
+       jmp memdet_looptop
+
+memdet_done:
+       ret
+
+memdet_fail:
+       # if size > 0, then it's not a failure, just the end
+       cmpl $0, boot_mem_map_size
+       jnz memdet_done
+
+       # just panic...
+       mov $memdet_fail_msg, %esi
+       call putstr
+0:     hlt
+       jmp 0b
+
+memdet_fail_msg: .asciz "Failed to detect available memory!\n"
+
+       .global boot_mem_map_size
+boot_mem_map_size: .long 0
+       .global boot_mem_map
+boot_mem_map: .space 128
+
+
 # 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.