diff --git a/Instruction-Set.md b/Instruction-Set.md
index 1eb7912..4c8b6ad 100644
--- a/Instruction-Set.md
+++ b/Instruction-Set.md
@@ -38,4 +38,19 @@
 *NEG instruction: CF is set to 0 if destination=0
 
 ### Instruction encoding
-WIP
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+Instructions are 1-3 words long and are fully defined by the first word.
+In a basic instruction, the lower 6 bits of the first word of the instruction
+are the opcode, and the remaining 12 bits are split into a 5 bit destination operand
+and a 5 bit source operand.
+In bits, a basic instruction has the format: `sssssdddddoooooo`
+
+Operand value:  
+
+| Description | Value |  
+| --- | --- |  
+| Invalid or none | `00000` |  
+| Immediate value at IP+1 | `11111` |  
+| [Immediate value at IP+1] | `11110` |  
+| Register | `00001`-`01000` |
+| [Register]  | `01001`-`10000` |
+| Register + Immediate value at IP+1 | `10001`-`11000` |
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