Esolang OISC:3

OISC:3 is a three word variant of OISC:4.  Naturally, we have to cut down on options.  It dispenses with the "if <=0, Jump" from every instruction.  Which, frankly, is no great loss.  I guess that makes this not technically a Subleq, but it keeps to the spirit of the thing.

Instruction A B C

If A>0:  Mem[C]=Mem[B]-Mem[A]
If A=0:  If Mem[B]<=0: Jump to C (Mem[|C|] if negative)

Negative addresses are indirect:  Mem[Mem[|A|]].

We can still use negative memory for data, but it has to be addressed indirectly.

Memory    Function
2             RET (set to NEXT before Jump)
1             NEXT (always IP+3)
0             IP (Initialized to 3) (HALT if negative)
-1           Input (replaces whichever A or B addressed it, -1 if none)
-2           Input Source and format
       (0 = wait for STDIN, 1 = immediate STDIN, 2... are Files)
-3           Output (outputs when a value is sent to it)
-4           Output Target and format
       (0 is STDOUT, 1 is STDERR, 2... are Files)
-5           Register a 
-6           Register b 
-7           Register c 
-8           Instruction Mode (IM) (activates Registers, then resets itself to 0)
-9...        Data