USB boot diagnostics continued

I disassembled the boot sector of my 4g USB drive with Ubuntu 10.04 LTS on it. The code below is a WIP so don't take it too literally, though it does match using "cmp" and "diff" though I used "hexdump≫ascii.txt" to use "diff" on the hex ascii output. I suppose eventually I will flash the BIOS to make a custom boot to my OS in ROM. It has a boot time of 1 second when running from BIOS ROM. I downloaded the XML file for "kate" which had the nasm highlighting and used that when generating the html for this post. I haven't really cleaned up the code well, but it seemed okay until I posted it and looked at all the vestigial progress mistakes of interpretation. In addition the code highlighting isn't perfect and I notice it highlighted ch at the end of CodeSwitch and ch in CheckBootable label , which seems an easy mistake to make, and "ndisasm" did something very odd with that code switch also and added a line with just -15 on it, which I suppose is some code glitch. I suppose the information at 0x1b8 is what the machine that wouldn't recognize my custom loader wanted or perhaps the partition table with a bootable partition, but I would suspect it is just the disk signature that it looks for, but I could be wrong and I will eventually discover what it really wants, even if I have to pull the BIOS code flash chip. I have written boot loaders about 100 times and I have done them for RISC and CISC as well as various other CPUs from the 70's, 80's and 90's. I also looked at the BIOS with

sudo dd if=/dev/mem of=bios.bin bs=65536 skip=15 count=1

;nasm -f bin -o mbr.img ipl.asm
;cat mbr.img > /dev/sdg1 ? USB?
;sudo dd if=mbr2.img of=/dev/sdg bs=512 count=1
        org 7C00h
Start:
  cli
  xor ax,ax
  mov ds,ax
  mov ss,ax
  mov sp,0x7c00
  mov si,sp
  push es
  push di
  push dx
  mov es,ax
  sti
  cld
  mov di,0x600
  mov cx,0x100
  rep movsw
;Move to 0:600 then jump there
jmp word 0:ReEntry-$$+0x600 

ReEntry:

  push dx
  mov ah,0x41
  mov bx,0x55aa
  xor cx,cx
  xor dh,dh
  stc
  int 0x13
  jc BiosOk
  cmp bx,0xaa55
  jnz BiosOk
  shr cx,1
  jnc BiosOk
  mov dword [BootCodeSwitch-$$+0x600],0x15eb42b4 ;-15?? nasm
BiosOk:
  pop dx
  mov ah,0x8
  int 0x13
  and cx,byte +0x3f
  push cx
  movzx ax,dh
  inc ax
  mul cx
  push dx
  push ax
  xor eax,eax
  cdq
  call PartitionLevel
Step5b:
  call StringPrint
db "Missing operating system.",13,10

LoadBootSector:
  pushad
  xor edx,edx
  mov bx,0x7c00
  push edx
  push eax
  push es
  push bx
  push byte +0x1
  push byte +0x10
  mov si,sp
BootCodeSwitch:
;mov ah,0x42
;jmp short $+0x17
;jmp short Alternate
  div dword [0x7bf4]
  shl ah,0x6
  mov cl,ah
  mov ch,al
  xchg ax,dx
  div byte [0x7bf8]
  mov dh,al
  or cl,ah
  inc cx
Alternate:
  mov ax,0x201
  mov dl,[0x7bfa]
  int 0x13
  add sp,byte +0x10
  popad
  ret
FunctionB2:
  call LoadBootSector
  mov  si,0x7dbe
  mov  di,0x7be
  mov  cx,0x20
  rep  movsw
  ret

PartitionLevel:
  pushad
  mov bp,sp
  mov bx,PartTable-$$+600h ;0x7be
  mov cx,0x4
IdentifyBootPartition:
  xor ax,ax
  push bx
  push cx
CheckBootable:
  test byte [bx],0x80
  jz NotBoot
  inc ax
  mov si,bx
NotBoot:
  add bx,byte +0x10
  loop CheckBootable
  dec ax
  jz double
  jns Error_Multiple_Partitions
  pop cx
  pop bx
PartitionLoop:
  mov al,[bx+0x4]
  cmp al,0xf
  jz TypeF
  and al,0x7f
  cmp al,0x5
  jnz NextPartition
TypeF:
  mov eax,[bx+0x8]
  mov edx,[bp+0x14]
  add eax,edx
  and edx,edx
  jnz Recurse
  mov edx,eax
Recurse:
  call FunctionB2
  jc BootErr2
  call PartitionLevel
BootErr2:
  mov eax,[bp+0x1c]
LocalB2:
  call FunctionB2
NextPartition:
  add bx,byte +0x10
  loop PartitionLoop
  popad
  ret
Error_Multiple_Partitions:
  call StringPrint
  db "Multiple active partitions.",13,10

double:
  mov eax,[si+0x8]
  add eax,[bp+0x1c]
  mov [si+0x8],eax
  call LoadBootSector
  jc Error3
  cmp word [0x7dfe],0xaa55
  jnz Step5b
  mov sp,0x7bfa
  pop dx
  pop di
  pop es
  cli
  jmp sp

Error3:
  call StringPrint
  Message1 db "Operating system load error.",13,10

StringPrint:
  pop si
CharOutLoop:
  lodsb
  mov ah,0xe
  mov bh,[0x462]
  mov bl,0x7
  int 0x10
  cmp al,0xa
  jnz CharOutLoop
  int 0x18
Stop:
  hlt
  jmp short Stop
;Disk signature?

times (01b8h  - ($ - $$))  db 0h    ;Zerofill up to 510 bytes
 db 0x98,0x93,8,0,0,0
;mov ah,0x42
;jmp short $+0x17
bootable equ 0x80
linuxPart equ 0x0c
start_sector equ 0x01f80 
total_sectors equ 0x0ef4080 
PartTable db bootable
   db 1   ;Head
   db 0xc   ;Sector and Cyl mixed
   db 0xf   ;0-7 cylinder    CHS start
   db linuxPart
   db 0x4,0xe0,0xa0  ;CHS last same
   dd start_sector
   dd total_sectors
times (01feh  - ($ - $$))  db 0h    ;Zerofill up to 510 bytes
 dw 0xaa55
;Sector2:
; db "Now is the time",13,10