Q2

A home-brew 12-bit mini-computer.

         __       __        
        /\ \     /\ \       000 | LDA | Z | A = [X]        
       /  \ \   /  \ \      001 | NOR | Z | A = A NOR [X] 
      / /\ \ \ / /\ \ \     010 | ADD | C | A = A + [X] 
     / / /\ \ \\/_/\ \ \    011 | SHR | C | A = [X] >> 1  
    / / /  \ \_\   / / /    100 | LEA | - | A = X 
   / / / __/ / /  / / /     101 | STA | - | [X] = A       
  / / / /\ \/ /  / / / __   110 | JMP | - | P = X
 / / /__\ \ \/  / / /_/\_\  111 | JFC | - | if !F: P = X  
/ / /____\ \ \ / /_____/ /   |
\/________\_\/ \________/   OOO D Z XXXXXXX - Operand
 joewing.net                     \ \_________ Zero Page
                                  \__________ Dereference

There are currently two hardware implementations of the Q2 instruction set:

• Q2 - The bit-serial single-board discrete transistor implementation.
• Q2A - A faster, parallel implementation using ATF16V8 PLDs.

Compared to the DEC PDP-8, the original Q2 would be like the PDP-8/S and the Q2A would be like the PDP-8/I.

This repo contains the following subdirectories:

• hardware - Hardware realizations.
  • q2 - The original discrete transistor Q2.
    • hdl - A Verilog model and test bench for simulating Q2 programs.
    • scad - An OpenSCAD 3d model for the case.
    • pcb - Schematics and PCB in KiCad
  • q2a - Programmable logic device implementation
    • pld - PLD files for programming the ATF16V8s and Verilog test bench
    • scad - An OpenSCAD 3d model for the case.
    • pcb - Schematics and PCB in KiCad
• q2asm - A Q2 assembler (in Rust).
• q2lc - A compiler for a simple language (in Rust).
• q2prog - A Q2 programmer (in Rust) for a Raspberry Pi (this is for the Q2 only; the Q2A requires an EEPROM programmer).
• examples - Q2 example programs (assembly and Q2L).

See joewing.net/projects/q2 for more information.

q2lc

q2lc is a compiler for Q2 programs. It is built using 'cargo build' in the q2lc directory.

The Makefile will take care of compiling and assembling Q2L programs in the examples directory automatically:

make examples/sieve.lst

This will compile sieve.q2l into sieve.q2 and then assemble sieve.q2 using q2asm to generate sieve.lst along with sieve.q2p.

q2asm

q2asm is the assembler for Q2 programs. It is built using 'cargo build' in the q2asm directory.

The Makefile will take care of assembling programs in the examples directory automatically:

make examples/hello.lst

This will generate the listing file for examples/hello.q2 as examples/hello.lst.

Instruction Set

All instructions are 1 word with the following format:

  FFF D Z XXXXXXX
   \  \ \    \____ Operand
    \  \ \________ Zero-Page    
     \  \_________ Dereference 
      \___________ Opcode

Instruction summary:

Opcode	Name	F	Description
000	LDA	Z	A = [X]
001	NOR	Z	A = A NOR [X]
010	ADD	C	A = A + [X]
011	SHR	C	A = [X] >> 1
100	LEA	-	A = X
101	STA	-	[X] = A
110	JMP	-	P = X
111	JFC	-	if !F, P = X

The flag is set if carry ('C'), zero ('Z'), or left unchanged ('-').

The following address modes are supported:

Value	DZ	Meaning
x	00	current-page relative
@x	10	indirect through current page
=x	01	zero-page relative
@=x	11	indirect through zero page

For convenience, the assembler also supports two additional address modes:

Value	DZ	Meaning
#x	00	immediate
#@x	10	immediate indirect

These cause an extra word to be allocated at the end of the current page for an immediate value.

Examples

; Negate: A = -A
  nor   #0
  add   #1

; Subtract: A - v
  nor   #0
  add   v       ; A = v - A - 1
  nor   #0      ; A = A - v

; Decrement A
  add   #-1

; NOT: A = ~A
  nor   #0

; OR: A = A | v
  nor   v
  nor   #0

; AND: A = A & v
  nor   #0
  sta   =t0   ; t0 = ~A
  lda   v
  nor   #0    ; A = ~v
  nor   =t0   ; A = ~(~A | ~v) = A & v

; Jump if F set
  jfc   $+2
  jmp   label

; Jump if a >= b
  lda   a
  nor   #0
  add   b
  jfc   ge

; Jump if a != b
  lda   a
  nor   #0
  add   b
  nor   #0
  jfc   ne

; Function call:
  lea   $+2   ; A = return address
  jmp   func

;...
func:
  sta   =ra   ; Save return address
  ; ...
  jmp   @=ra  ; Return

; Long jump
  jmp   @#addr

; Long call
  lea   $+2
  jmp   @#addr

; Rotate right function
; x0 >>> 1
ror:
  sta   =ra       ; save return address
  shr   =x0       ; A = x0 >> 1
  jfc   @=ra      ; return if no carry
  add   0x800
  jmp   @=ra

; Push A on to the stack
  sta   @=sp
  lda   =sp
  add   #-1
  sta   =sp

; Pop A off the stack
  lea   =1
  add   =sp
  sta   =sp
  lda   @=sp

I/O

I/O is handled through address 0xFFF.

LCD

Writes are directed to the LCD when the top 3 bits written to 0xFFF are zero. Writes to the LCD can be either commands or data. The low 8-bits are sent directly to the LCD and the 9th bit determines if data (0) or a command (1) is to be sent. Reads are not supported.

Buttons

There are 5 buttons. Reading from 0xFFF returns the state of the buttons in the low five bits where 0 is returned if the button is pressed and 1 is returned if the button is not pressed.

I2C

The SCL and SDA bits can be set by writing to 0xFFF with the top bit set. Bit 9 controls SCL and bit 8 controls SDA. Reads from 0xFFF return the state of the SCL and SDA bits.

Memory Extension

The memory extension device allows the Q2 to address more than 4096 words of memory. This is accomplished through the use of two additional latches: the data field (DF) and the instruction field (IF). Memory accesses for indirect loads or stores have the data field appended to the address. All other accesses have the instruction field appended.

On reset, both the data and instruction fields are set to 0. The data field can be set by writing 0xCxx to address 0xFFF (note that the field is ignored for I/O). This will set the data field to xx. The data field is copied to the instruction field when a jump takes place.

Summary

Writes:

• 00 0 C DDDDDDDD - LCD
• 10 C D 00000000 - I2C (C for SCL, D for SDA)
• 11 0 0 0000000F - Field

Reads:

• 11 C D 111KKKKK