diff --git a/doc/isa.org b/doc/isa.org
index a81d714..7a36b8e 100644
--- a/doc/isa.org
+++ b/doc/isa.org
@@ -4,10 +4,11 @@
 ** Registers
 
 - PC :: Program counter (32bit)
-- AC :: Accumulator (32bit)
-- X  :: X register (32bit)
-- Y  :: Y register (32bit)
-- SR :: status register (32bit)
+- A  :: Accumulator (32bit)
+- D  :: Direct register (32bit)
+- X  :: X index register (32bit)
+- Y  :: Y index register (32bit)
+- SR :: status register (8bit)
 - SP :: stack pointer (32bit)
 
 Status register flags