Viktor's amazing 4-bit processor

vpsmall.jpgBeware of programmers who carry screwdrivers. - Leonard Brandwein

I've been a software guy all my life. However, I was always fascinated by electronics, and part of my success as a programmer was due to my thorough understanding of how computers work.

Back in 1999, I put that understanding to the ultimate test: I actually designed, and built, a simple but functional 4-bit computer from low-level electronic components (TTL logic gates.) Although this machine has less than one tenth of a percent of the speed and one millionth of the memory of a modern Pentium system, not to mention that its "user interface" is just a set of miniature switches and blinking LED lights, I still consider this a proud accomplishment.

If you are insterested, you can study a full description of this system, including logic diagrams and more. Or, if you just want an overview, here's a photo and, to demonstrate the programming model of this processor, a small program that multiplies two 4-bit numbers to obtain an 8-bit result:

00:  F          CLF       ; Initialize
01:  7 0 0      AND 00    ; Load 0 to accumulator
04:  2 E F      STA FE    ; Set #FE to 0
07:  1 C F      LDA FC    ; Load first argument
0A:  5 F 0      AND 0F    ; Take low nybble
0D:  2 A F      STA FA    ; Store at working storage
10:  1 D F      LDA FD    ; Load second argument
13:  5 F 0      AND 0F    ; Take low nybble
16:  2 9 2      STA 29    ; Store as argument to ADD instruction
19:  1 A F      LDA FA    ; Load first argument
1C:  F          CLF       ;
1D:  E          ROR       ; Take one bit
1E:  2 A F      STA FA    ; Store the rest
21:  C E 2      JNC 2E    ; If not 1, no need to add
24:  1 E F      LDA FE    ; Load result
27:  F          CLF       ;
28:  7 0 0      ADD 00    ; Add second argument (stored here)
2B:  2 E F      STA FE    ; Store result
2E:  1 9 2      LDA 29    ; Load second argument
31:  F          CLF       ; Multiply by 2
32:  D          ROL       ;
33:  2 9 2      STA 29    ; Store
36:  1 A F      LDA FA    ; Reload first
39:  F          CLF       ;
3C:  7 0 0      ADD 00    ; Check for 0
3D:  9 D 1      JNZ 1D    ; Repeat loop if necessary
40:  0          HLT       ; Result obtained
41:  3 0 0      JMP 00    ; Restart

FA: 0 0                   ; Working storage
FC: 3 5                   ; Operands
FE: 0 0                   ; Result

Complex floating point algorithms, such as that of the Gamma function (my favorite programming example for programmable calculators) are probably forever beyond the capabilities of this beast; you're not going to implement a floating point library in a mere 256 nybbles! It is, however, possible to implement a simple factorial program, and surprisingly, it doesn't even require the use of a multiplication subroutine. It makes efficient use of repeated additions instead. This implementation calculates an 8-bit result, so the highest number it accepts as its argument is 5, but it still demonstrates a neat solution. After entering the program, its argument must also be entered at address FE-FF; the 1-byte (2-nybble) result can be read from FC-FD.

00:  1 E F      LDA  FE   ; Move FE to FC (argument in FE)
03:  2 C F      STA  FC
06:  1 C F  L1: LDA  FC   ; Move FC to 35
09:  2 5 3      STA  L5+1
0C:  1 E F      LDA  FE   ; Decrement FE
0F:  F          CLF
10:  8 1 0      SUB  01
13:  9 A 1      JNZ  L3   ; If FE==0 END
16:  0          HLT       ; (result is in FC)
17:  3 6 1  L0: JMP  L0
1A:  2 E F  L3: STA  FE
1D:  2 A F      STA  FA   ; Move FE to FA
20:  1 A F  L2: LDA  FA   ; Decrement FA
23:  F          CLF
24:  8 1 0      SUB  01
27:  9 D 2      JNZ  L4   ; If FA==0 goto L1
2A:  3 6 0      JMP  L1
2D:  2 A F  L4: STA  FA
30:  1 C F      LDA  FC   ; FC=FC+35
33:  F          CLF
34:  7 0 0  L5: ADD  00
37:  2 C F      STA  FC
3A:  3 0 2      JMP  L2   ; LOOP

Disclaimer: I built this processor as a hobby project, as a means of self-education to learn more about hardware architectures. The ideas and inventions used here are my own, but it is entirely possible that some of the solutions have been used by others elsewhere, and are protected by patents. If you plan to utilize any of the solutions disclosed herein for a commercial purpose, you are advised to do the necessary research to ensure that your product or service does not violate an in-force patent in your country. Needless to say, this disclaimer does not free you from the obligation to observe my copyright prior to republishing my work in whole or in part, or otherwise utilizing it for a commercial purpose.