EFF file format

Applies to:
BG1: TotS, BG2, BG2: ToB, IWD, IWD:HoW, IWD:TotL

General Description
This file format describes an effect (opcode) and its parameters. The format is an extended version of that found embedded in creatures, items and spells, and is a replacement for the version 1 embedded effects used in BG1. The engine appears to roll a probability for each valid target type, rather than one probability per attack.

Overall structure:


Offset Size (data type) Description
0x0000 4 (char array) Signature ('EFF ')
0x0004 4 (char array) Version ('V2.0')


When an effect is called via an eff, several of the fields (including duration and timing mode) are overriden by the calling SPL/ITM. Other fields are copied directly from the calling SPL/ITEM and overwrite the values defined in the EFF.

Offset Size (data type) Description
0x0008 4 (char array) External EFFs - the field is the same as the signature field from the header
Embedded EFFs - this field is zeroed out
0x000c 4 (char array) External EFFs - the field is the same as the version field from the header
Embedded EFFs - this field is zeroed out
0x0010 4 (dword) Opcode number
0x0014 4 (dword) Target type
  • 0=None
  • 1=Self (pre-projectile)
  • 2=Pre-target
  • 3=Party
  • 4=Everyone (inc. party)
  • 5=Everyone (excl. party)
  • 6=Everyone matching specific value of caster (or Party if cast by party member)
  • 7=Everyone matching specific value of target
  • 8=Everyone (excl. caster)
  • 9=Self (post-projectile)
0x0018 4 (dword) Power
0x001c 4 (dword) Parameter 1
0x0020 4 (dword) Parameter 2
0x0024 2 (word) Timing mode
  • 0=Duration
  • 1=Permanent
  • 2=While equipped
  • 3=Delayed Duration
  • 4=Delayed
  • 5=Delayed (transforms to 8)
  • 6=Duration?
  • 7=Permanent?
  • 8=Permanent (unsaved)
  • 9=Permanent (after Death)
  • 10=Trigger
  • 4096=Absolute duration
0x0026 2 (word) Timing ?
0x0028 4 (dword) Duration
0x002c 2 (word) Probability 1
0x002e 2 (word) Probability 2

Note: The BG1 engine treats probability as a single word, effects are applied in the range 0-probability.
Note: Other engines treat probability as two values, effects are applied in the range probability <= x <= probability.
Probability range is 0-99
0x0030 8 (resref) Resource
0x0038 4 (dword) Dice Thrown
0x003c 4 (dword) Dice Sides
0x0040 4 (dword) Saving throw type
  • bit 0: Spells
  • bit 1: Breath
  • bit 2: Death
  • bit 3: Wands
  • bit 4: Polymorph
NB. If more than 1 saving throw type is specified, the target rolls against their best save
0x0044 4 (dword) Save bonus
0x0048 4 (dword) Set local variable only if non-existant
0x004c 4 (dword) Primary type (school)
0x0050 4 (dword) Unknown
0x0054 4 (dword) Parent resource lowest affected level
0x0058 4 (dword) Parent resource highest affected level
0x005c 4 (dword) Resistance

The default behaviour is that effects cannot be dispelled and ignore resistance.
  • bit 0: Can be dispelled, affected by resistance
  • bit 1: Ignores resistance (when used in combination with bit 0)

This can also be presented in the more traditional way:
  • 0=Nonmagical
  • 1=Can be dispelled/Affected by resistance
  • 2=Cannot be dispelled/Ignores resistance
  • 3=Can be dispelled/Ignores resistance
0x0060 4 (dword) Parameter 3
0x0064 4 (dword) Parameter 4
0x0068 4 (dword) Parameter 5
0x006c 4 (dword) Parameter 6
0x0070 8 (resref) Resource 2 (VVC in many effects)
0x0078 8 (resref) Resource 3
0x0080 4 (dword) Caster X coordinate
0x0084 4 (dword) Caster Y coordinate
0x0088 4 (dword) Target X coordinate
0x008c 4 (dword) Target Y coordinate
0x0090 4 (dword) Parent resource type (0=None, 1=Spell, 2=Item)
0x0094 8 (resref) Parent resource
0x009c 4 (dword) Parent resource flags (from SPL 0x18 or ITM ext header 0x26)
0x00a0 4 (dword) Projectile
0x00a4 4 (dword) Parent resource slot
0x00a8 32 (bytes) Variable
0x00c8 4 (dword) Caster level
0x00cc 4 (dword) FirstApply (internal flag)
0x00d0 4 (dword) Secondary Type
0x00d4 4*15 (dword) Unknown