Instruction Set Shuffle

Having ADD and SUB in different op code pages seems wrong, as SUB (ACC = Value - 1) can be coded as:

• LOAD Value
• XOR F
• ADD 1
• XOR F

Yes, the carry flag works.

The current op code set would be:

• LOAD Value
• PAGE 8
• SUB 1
• PAGE 0

No saving!

To test for a value you could use:

• LOAD Value
• XOR F
• ADD Test
• XOR F
• ADD F
• JNC [Value == Test]
• [Value != Test]

Or:

• LOAD Value
• SUB Test
• ADD F
• JNC [Value == Test]
• [Value != Test]

A better set of op codes would be:

• 0-1  LOAD/LOAD
• 2-3  ADD/SHR using Carry
• 4-5  SUB/XOR
• 6-7  AND/OR 
• 8-9  JNC
• A-B  STORE
• C-D  READ
• E-F  PAGE

AND has been promoted over NAND as it can test for bit states:

• LOAD Value
• AND Bit Mask
• ADD F
• JNC [False == 0]
• [True != 0]

Also:

• ADD 0 clear the Carry flag
• SUB 0 sets the Carry flag.

This will be version 5.

Here is the simulation of up counting followed by down counting, then repeat:

The code for the animation is:

E0  PAGE 0           ; Select Op Code Set 0
20  ADD 0            ; Clear Carry
Repeat:
00  LOAD 0           ; Clear ACC
Loop1:
AF  SAVE F           ; Output ACC
A0  SAVE MEM[0]      ; Save to RAM
21  ADD 1            ; Increment
83  JNC 3            ; Loop1
20  ADD 0            ; Clear Carry
0F  LOAD F           ; Set F
Loop2:  
AF  SAVE F           ; Output ACC
A0  SAVE MEM[0]      ; Save to RAM
41  SUB 1            ; Decrement
89  JNC 9            ; Loop2
20  ADD 0            ; Clear Carry
82  JNC 2            ; Repeat

The top level:

The ALU:

And control:

AlanX