DCR instruction doesn't work properly after 80h

[torishabhmehta]

I was working with a pretty basic multiplication code when I encountoured that my code is failing for higher values of a particular operand. After investigating, I found that the dcr operation sets the S flag to 1 for values in register higher than 80h. This is obviously a bug, since S flag should be set only if dcr is operating on 00h register entry. Please look into this. Here's my code for error reproduction:

;<Multiplication Program>

jmp load

;data
num: db 0ffh, 81h
res: db 00h, 00h

;code
load: nop
lhld num
xchg
mov c, d
mvi d, 0h
lxi h, 0000h


main: nop
dcr c
jm end
dad d
jmp main

end: nop
shld res
hlt

[torishabhmehta]

The code works perfectly for values in num+1 less than/equal to 80h. The dcr operation acting on higher values than that simply sets the S flag, which triggers the jm command