Notes

• Located at the start of real memory (0000:0000 through 0000:0x3FF)
• 256 entries - each is a complete 32 bit address (segment and offset)
• no code or data can go here!
• some are for DOS (mostly all through INT 0x21), some are for the BIOS (around 10)
• populated by OS
• The INT instruction tells CPU to push the next IP on the stack, fetch the address stored in the 33rd (0x21) vector slot, & jump to that code
• The IRET pops the return address off the stack and returns to the calling program

Some popular DOS entries

• INT 21H - service dispatcher for > 50 DOS service routines
  • primitive text to screen - ah = 0x09 (see MyFirstDOSAssemblyProgram)
  • terminate current program - ah = 0x4c (also used in MyFirstDOSAssemblyProgram)

Some popular BIOS entries

• 0x10h - video display
  • position cursor - ah = 2, bh = 0 (display page), dl = x, dh = y
  • clear / scroll region - ah = 6, al = 2, cx/dx = (upper left/lower right xy's of region)
• 0x16h - keyboard
• 0x17h - printer

-- MattWalsh - 17 Apr 2004