diff options
| -rw-r--r-- | baremetal/boot.hex | 34 |
1 files changed, 16 insertions, 18 deletions
diff --git a/baremetal/boot.hex b/baremetal/boot.hex index fda3c287..45b4f4d8 100644 --- a/baremetal/boot.hex +++ b/baremetal/boot.hex @@ -59,6 +59,7 @@ # see baremetal/120allocate.subx # stack grows down from 0x00070000 # see below +# Consult https://wiki.osdev.org/Memory_Map_(x86) before modifying any of this. ## 16-bit entry point @@ -81,17 +82,15 @@ 8e e0 # fs <- ax 8e e8 # gs <- ax - # We don't read or write the stack before we get to 32-bit mode, but we - # still need to move the stack in case BIOS initializes it in some low - # address that we want to write code into. - # - # We'll grow the stack downward from somewhere in 0x0007xxxx; - # consult https://wiki.osdev.org/Memory_Map_(x86) when modifying this + # initialize stack to 0x00070000 + # We don't read or write the stack before we get to 32-bit mode, but BIOS + # calls do. We need to move the stack in case BIOS initializes it to some + # low address that we want to write code into. b8 00 70 # ax <- 0x7000 8e d0 # ss <- ax - # Not bothering initializing sp register. Que sera sera. + bc 00 00 # sp <- 0x0000 -# 11: +# 14: # disk read #1: load remaining sectors from first two tracks of disk into addresses [0x7e00, 0x17800) b4 02 # ah <- 2 # read sectors from disk # dl comes conveniently initialized at boot time with the index of the device being booted @@ -102,12 +101,11 @@ # address to write sectors to = es:bx = 0x7e00, contiguous with boot segment bb 00 00 # bx <- 0 8e c3 # es <- bx -# 20: bb 00 7e # bx <- 0x7e00 [label] cd 13 # int 13h, BIOS disk service - 0f 82 a6 00 # jump-if-carry disk_error [label] + 0f 82 a3 00 # jump-if-carry disk_error [label] -# 29: +# 2c: # undo the A20 hack: https://en.wikipedia.org/wiki/A20_line # this is from https://github.com/mit-pdos/xv6-public/blob/master/bootasm.S # seta20.1: @@ -118,7 +116,7 @@ b0 d1 # al <- 0xd1 e6 64 # port 0x64 <- al -# 33: +# 36: # seta20.2: e4 64 # al <- port 0x64 a8 02 # set zf if bit 1 (second-least significant) is not set @@ -127,7 +125,7 @@ b0 df # al <- 0xdf e6 64 # port 0x64 <- al -# 3d: +# 40: # adjust video mode b4 4f # ah <- 4f (VBE) b0 02 # al <- 02 (set video mode) @@ -136,7 +134,7 @@ # fallback mode: 0x0101 (640x480x256) cd 10 # int 10h, Vesa BIOS extensions -# 46: +# 49: # load information for the (hopefully) current video mode # mostly just for the address to the linear frame buffer b4 4f # ah <- 4f (VBE) @@ -145,7 +143,7 @@ bf 00 7f # di <- 0x7f00 (video mode info) [label] cd 10 -# 52: +# 55: # switch to 32-bit mode 0f 01 16 # lgdt 00/mod/indirect 010/subop 110/rm/use-disp16 f8 7c # *gdt_descriptor [label] @@ -155,8 +153,8 @@ ea 00 7d 08 00 # far jump to initialize_32bit_mode after setting cs to the record at offset 8 in the gdt (gdt_code) [label] # padding -# 66: - 00 00 00 00 00 00 00 00 00 00 +# 69: + 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 @@ -227,7 +225,7 @@ e9 fd ff # loop forever 8e e8 # gs <- ax # 10e: - bc 00 00 07 00 # esp <- 0x00070000 (wasting earlier stack above) + bc 00 00 07 00 # esp <- 0x00070000 # 113: # load interrupt handlers |