That's "Sign EXtension" for the those wondering about that. The A and B registers combined to be the 16-bit D register, and this instruction would be used to c

That's "Sign EXtension" for the those wondering about that. The A and B registers combined to be the 16-bit D register, and this instruction would be used to convert an 8-bit signed integer to 16-bit. Also amusing was the "BRanch Always" instruction, which is the basic unconditional jump. There were branch instructions for all the usual conditions of the most recent operation such as 'overflow' during an addition. All the branch operations used relative addresses, the two 'jump' instructions used absolute addresses. So it was easy to write relocatable code on the 6809. This is binary code that can just be moved to another location in memory and still execute properly, without needing to be re-linked.