This is a cosmetic bug.
Simply start s51 with the most basic processor (might have to use -t8051 as a command-line option)
Then in the sim execute the step command once.
On the stack pointer line, it begins with "SP 0x100000007" but a 0x100000007 value or address cannot exist in a standard 8051. The value should be changed to 0x07 as 7h is the default address location of the stack pointer.
I attached a picture so you have a better understanding of what I mean along with the proposed change to the output screen should someone else execute commands that don't modify the stack pointer.
Fixed in [r13107]