# Jump Logic

A project log for Hardware Controlled 4 bit CPU

A hardware controlled 4 bit CPU based on lessons learnt from the TD4 and the CHUMP CPUs

agp.cooper 12/18/2022 at 00:260 Comments

Jump Logic

The current jump logic uses JNC (Jump on Not Carry). It is a complete system but not that intuitive. For multi-nimble/byte arithmetic it is straight forward, if no carry adjustment is required (i.e. not carry), skip the add carry code.

A JGE (i.e. Jump on Greater or Equal) can be constructed by complementing one of the operands, and adding the other operand.  Following is an example of testing the input for a program number  1 to 6:

```Address    OpCode    Const    Comment    CODE
0    C    F    READ F
1    3    F    LOAD M (PORT)
2    0    0    CLEAR CARRY
#    TEST PROGRAM 6
3    0    A    ADD 10         # NOT 5
4    8    7    JNC 7          # JGE 5
5    E    7    SET PAGE       # JUMP PROGRAM 6
6    8    0    JMP ADDR
#    TEST PROGRAM 5
7    0    1    ADD 1          # NOT 4
8    8    B    JNC B          # JGE 4
9    E    6    SET PAGE       # JUMP PROGRAM 5
A    8    0    JMP ADDR
#    TEST PROGRAM 4
B    0    1    ADD 1          # NOT 3
C    8    F    JNC F          # JGE 3
D    E    5    SET PAGE       # JUMP PROGRAM 4
E    8    0    JMP ADDR
#    SET NEW PAGE 1
F    E    1    PAGE 1
#     TEST PROGRAM 3
10    0    1    ADD 1         # NOT 2
11    8    4    JNC 4         # JGE 2
12    E    4    SET PAGE      # JUMP PROGRAM 3
13    8    0    JMP ADDR
#     TEST PROGRAM 2
14    0    1    ADD 1         # NOT 1
15    8    8    JNC 8         # JGE 1
16    E    3    SET PAGE      # JUMP PROGRAM 2
17    8    0    JMP ADDR
#     TEST PROGRAM 1
18    0    1    ADD 1         # NOT 0
19    8    C    JNC C         # JGE 0
1A    E    2    PAGE 2        # JUMP PROGRAM 1
1B    8    0    ADDR 0
#     RETURN - NO PROGRAM SELECTED
1C    E    0    SET PAGE 0
1D    8    0    JMP ADDR 0
1E    0    0    NOP
1F    0    0    NOP
```

One option is to add a comparator to the ALU and to use one of the flags (i.e. A<B, A=B and A>B) or the compliment to trigger the jump. It would have the advantage of not altering the accummulator.

---

I have been thinking more about this. It would be an efficient and useful to have a "TEST" register but it requires freeing up an op code. XOR being the one to use:

But rather than doing that I am thinking of an op code to select an alternate op code set.

PAGE and READ are similar so they could share the same op code, and the PAGE slot used to set alternate op codes:

• NAND/NOR
• XOR/XOR?
• JNC/JGE?
• STORE/STORE?