At the MS-DOS prompt, type "spear debugmode". When the game begins, hold Shift-Alt-Backspace. A message should confirm your success. Congratulations! You may now use any of the following codes while playing.

TAB G    God mode
TAB I    Extra items
TAB N    Walk through walls
TAB B    Change border
TAB W    Warp to any level 
TAB E    End level
TAB S    Slow motion
TAB P    Pause

While playing, press the keys "I", "L" and "M" at the same time. This code will give you 100% life, 99 ammo, both keys, and all of the guns. But beware: this code takes away your score too.

To access God Mode press Tab, g and f10 at the same time. Do this while playing.