I don't want to put this in the Optimized Routines thread as I haven't had it long enough for it to be ultra-optimized.

This code is intended as a method of testing if HL is divisible by DE. It's average run time for random 16-bit inputs is 163cc, and where DE<=HL (more likely in practice), it is 229cc. For a code that is doing prime-checking, it will more likely take about 707.5cc (Since DE will be roughly the square root of HL or less).
isDivisible:
;;Inputs: DE,HL
;;Outputs: c flag set if HL is not divisible by DE, else c flag is set.
;;         HL is 0 if true.
;;1087cc worst case (HL=65535, DE=1).
;;Average for random inputs: 163cc
;;Average for DE<=HL: 229cc.
    ld a,d \ or e \ ccf \ ret z         ;remove this if DE is always guaranteed non-zero
;step 1
    ld a,e \ or l \ rra \ jr c,step2    ;\
    srl d \ rr e \ rr h \ rr l          ; |
    ld a,e \ or l \ rra \ jr nc,$-11    ; |Remove these if DE is always guaranteed odd at input.
step2:                                  ; |
    ld a,e \ rra \ ccf \ ret c          ;/
;steps 3, 4, and 5
    ld a,l
    or a
loop:
    sbc hl,de \ ret c \ ret z
    rr h \ rra \ bit 0,a \ jr z,$-5
    ld l,a
    jp loop

Motivation and Development
I often find myself in a situation where I need to find the factors of a number, but I have no technology around to aid me. This means I need to use... mental arithmetic! I've been doing this for 15 years, so I have refined my mental process quite a bit. It is still a trial division algorithm, but with a very obfuscated "division" technique. We don't need to do 1131/7 to see if it is divisible by 7, we just need to see if 7 divides 1131 and this is what my algorithm does. Interestingly, testing divisibility at the algorithmic level is a little faster than division. Not by much, but it is also non-negligible.
The Algorithm
The core algorithm is designed around checking that (A mod B == 0) is true or false. We also make the assumption that B is odd and by extension, non-zero. The case where B is non-zero and even will be discussed later.

Since B is odd, 2 does not divide B. This means that if A is even:
    (A mod B == 0) if and only if  (A/2 mod B)==0.
We also know by the definition of divisibility that
    (A mod B) == (A+c*B mod B)
where c is any integer. Combining all this, we have an algorithm:

• Remove all factors of 2 from A
• With A now odd, do A=A-B
  
  • If the result is zero, that means (A mod B == 0)
  • If the result underflow (becomes "negative", or on the Z80, sets the carry flag), it means that A was somewhere on [1,B-1], so it is not divisible by B.
• Continue back at 1.

Now suppose B is allowed to be non-zero and even. Then B is of the form d*2^k where d is odd. This just means there are some factors of 2 that can be removed from B until it is odd. The only way A is divisible by B, is if it has the same number or more of factors of 2 as B. If we factor out common factors of 2 and find B is still even, then A is not divisible by B. Otherwise we have an odd number and only need to check the new (A mod d) for which we can use the "odd algorithm" above.
So putting it all together:

• If B==0, return FALSE.
• Remove common factors of 2 from A and B.
• If B is even, return FALSE.
• Remove all factors of 2 from A.
• Subtract B from A (A=A-B).
  
  • If the result is zero, return TRUE.
    
  • If the result is "negative" (setting the carry flag on many processors), return FALSE.
• Repeat at 4.

The overhead steps are (1) to (3). The iterated steps are (4) and (5).
Because (5) always produces an even number, when it then performs step 4, it always divides by at least one factor of 2. This means the algorithm takes at most 1+ceil(log2(A))-floor(log2(B) iterations. For example, if A is a 37-bit number and B is a 13-bit number,this takes at most 38-13 = 25 iterations. However, in practice it is usually fewer iterations.
Example Time:
Say I wanted to test if 1337 is divisible by 17.
Since 17 is odd, we can proceed.
1337 is odd, so no factors of 2 to remove.
1337-17 == 1320.
1320/2 == 660
660/2 == 330
330/2 == 165
165-17 == 148
148/2 == 74
74/2 == 37
37-17 == 20
20/2 == 10
10/2 == 5
5-17 == -12
So 1337 is not divisible by 17.

Now test divisibility by 7:
1337 => 1330
=>665
=>658
=>329
=>322
=>161
=>154
=>77
=>70
=>35
=>28
=>14
=>7
=>0

So 1337 is divisible by 7.


The worst-case timing is 66*16+31 = 1087