diff --git a/registers.md b/registers.md
index c8c3ae8..fd23b05 100644
--- a/registers.md
+++ b/registers.md
@@ -18,15 +18,19 @@ signed integer is sign extended from an 8-bit signed integer and thus has the
 range -128 to 127.
 
 #### MBRU
-When read as MBRU, the register reads as a 32-bit *unsigned* integer. The 32-bit
-signed integer is extended with zeros from an 8-bit *unsigned* integer and thus
-has the range 0 to 255.
+When read as *Memory Buffer Register Unsigned*, the register reads as a 32-bit
+*unsigned* integer. The 32-bit signed integer is extended with zeros from an
+8-bit *unsigned* integer and thus has the range 0 to 255.
 
 ### OPC
 The *Old Program Counter* is used to store the old value of the **PC**-register,
 although it is very commonly used as a *scratch register* for storing other
 values.
 
+### MPC
+The *Micro-Program Counter* point so the current position in the microcode. It
+is automatically set after very operation.
+
 Read and write
 --------------
 
@@ -72,6 +76,7 @@ The *Hold* register is used to hold the left operand in the operations performed
 by the ALU.
 
 ### Z & N
-The *Z* and *N* registers are *virtual registers*. They are automatically set
-after any ALU operation. A MAL `if(x)` statement can only accept *Z* and *N* as
-input.
+The *Zero* and *Negative* registers are *virtual registers*. They are
+automatically set after any ALU operation. A MAL `if(x)` statement can only
+accept *Z* and *N* as input. *Z* is on if the output of the latest ALU operation
+was zero. *N* is on if the output was negative.