The Byte Data Processor 1 (BDP-1)

The BDP-1 is a simple 8-bit learning computer inspired by the Little Man Computer.

It is designed to complement the MTMC-16 as a simpler introduction to computing.

Architecture

• 8 bit words
• 4 general registers: A, B, C, D
• 256 bytes of memory
• 16 x 16 display, 8-bit color (RGB332)

Register conventions

• A - accumulator: destination of RMATH/RLOGIC; tested by JZ/JP/JN; result register for WINT/RINT/RAND
• B, C - general purpose, PLOT instruction uses as x and y coordinates (masked to 4 bits)
• D - general purpose, PLOT instruction uses as color

Instruction Set

• Top 4-bit nibble is the instruction type
• Bottom nibble is op code or args
• The Most Significant Bit (MSB) of the top nibble distinguishes single-word (0xxx) from double-word (1xxx) instructions
• The assembler uses a unified MOV dest, src x86-style syntax (Intel order):
  • Brackets mean memory access
  • Bare names/numbers are immediate values

Type	Top	Op	Assembly form	Size	Description
SYS	0000	0000	HALT	single	halts computer
SYS	0000	0001	WINT	single	writes A to standard out as an unsigned integer
SYS	0000	0010	RINT	single	reads standard in to A as an unsigned integer
SYS	0000	10RR	RAND RR	single	places a random number in register RR
SYS	0000	1111	NOOP	single	no operation
GFX	0001	0000	CLEAR	single	resets graphics to blank (all 0)
GFX	0001	0001	PLOT	single	sets pixel at position B, C to color in D
MOV	0010	SSDD	MOV DD, SS	single	DD := SS (register-to-register copy)
MOV	0011	PPDD	MOV DD, [PP]	single	DD := M[PP] (indirect load via pointer in PP)
MOV	0100	PPSS	MOV [PP], SS	single	M[PP] := SS (indirect store via pointer in PP)
RMATH	0101	00RR	ADD A, RR	single	A += RR (accumulator add)
RMATH	0101	01RR	SUB A, RR	single	A -= RR
RMATH	0101	10RR	MUL A, RR	single	A *= RR
RMATH	0101	11RR	DIV A, RR	single	A /= RR (integer; div by 0 -> 0)
RUNARY	0110	00RR	INC RR	single	RR += 1
RUNARY	0110	01RR	DEC RR	single	RR -= 1
RUNARY	0110	10RR	NEG RR	single	RR := -RR (two's complement)
RUNARY	0110	11RR	ZERO RR	single	RR := 0
RLOGIC	0111	00RR	AND A, RR	single	A &= RR
RLOGIC	0111	01RR	OR A, RR	single	A |= RR
RLOGIC	0111	10RR	XOR A, RR	single	A ^= RR
RLOGIC	0111	11RR	SHL A, RR	single	A <<= RR (shift left, fills 0)
MEM	1000	00RR	MOV RR, [addr]	double	RR := M[addr]
MEM	1000	01RR	MOV RR, imm	double	RR := imm (the second byte is the literal value)
MEM	1000	10RR	MOV [addr], RR	double	M[addr] := RR
MMATH	1001	00RR	ADD RR, [addr]	double	RR += M[addr]
MMATH	1001	01RR	SUB RR, [addr]	double	RR -= M[addr]
MMATH	1001	10RR	MUL RR, [addr]	double	RR *= M[addr]
MMATH	1001	11RR	DIV RR, [addr]	double	RR /= M[addr] (integer; div by 0 -> 0)
JUMP	1010	0000	JMP addr	double	PC := addr
JUMP	1010	0001	JZ addr	double	jump if A == 0
JUMP	1010	0010	JP addr	double	jump if A > 0 (high bit clear, value non-zero)
JUMP	1010	0011	JN addr	double	jump if A < 0 (high bit set)
JUMP	1010	0100	JSR addr	double	writes return address into M[addr], then jumps to addr+1
JUMP	1010	0101	JI addr	double	PC := M[addr] (indirect jump; serves as RET when paired with JSR)
LOGIC	1011	00RR	AND RR, [addr]	double	RR &= M[addr]
LOGIC	1011	01RR	OR RR, [addr]	double	RR |= M[addr]
LOGIC	1011	10RR	XOR RR, [addr]	double	RR ^= M[addr]
LOGIC	1011	11RR	SHL RR, [addr]	double	RR <<= M[addr] (shift left, fills 0)

Subroutine convention

The BDP uses a PDP style for invoking sub-routines:

• JSR slot writes the return address into M[slot] and jumps to slot+1
• The function body lives at slot+1
• Return with JI slot (indirect jump through the saved address)
• NO STACK: not re-entrant!

        MOV A, 5
        JSR double      ; M[double] := return addr; PC := double+1
        WINT
        HALT
double: SW 0             ; return slot; body starts at the next byte
        MOV [scratch], A
        ADD A, [scratch]
        JI  double       ; return
scratch: SW 0

Programming

The BDP-1 can be programmed three ways, from lowest-level to highest:

• Punch cards: click holes to toggle individual bits of each byte
• Assembly: mnemonics and labels, run through the built-in assembler
• ByteTran: a small Fortran-flavored HLL that compiles to BDP-1 assembly

Each is documented below.

Punch Cards

The CARD tab presents a vertical, 32-byte punch card.

Each row has eight clickable holes (bit 7 on the left, bit 0 on the right)

Clicking a hole flips the corresponding bit.

You can load the program into memory using the "Load Card" button below the card.

Note that this is not how punch card programming worked in the era we mostly think of: only very early punch card systems used raw binary encodings.

Assembly

The ASM tab is a plain text editor that runs through a two-pass assembler.

Click ASSEMBLE & LOAD to assemble the source & load In it into memory starting at 0x00. A

The EXAMPLES dropdown ships several short programs (random pixels, count loop, color sweep, JSR demo, indirect array sum, XOR moire).

ASM Guide

Write one instruction per line. Operands are separated by spaces or commas. Mnemonics and register names are case-insensitive.

        MOV A, 5         ; load immediate
        MOV B, [count]   ; load from memory
        ADD A, B         ; accumulator add (dst must be A)
        MOV [count], A   ; store back
        HALT
count:  SW 0

Operands

The same mnemonic (MOV, ADD, SUB, ...) accepts multiple operand shapes: the assembler picks the right encoding from the syntax

Operand	Meaning
A/B/C/D	a register
[A] etc.	memory at the address held in that register (indirect)
[label] / [42] / [0x2A]	memory at the given address (absolute)
label / 42 / 0x2A / 0b101 / 'X'	immediate value (or address, for jumps)

Brackets mean "memory access."

Bare names and numbers are immediate (for data ops) or jump targets (for JMP/JZ/JP/JN/JSR/JI).

Numeric literals can be decimal, 0x-prefixed hex or 0b-prefixed binary.

Assembler directives

• label: defines a label at the current address
• SW value [, value ...] emits one raw byte per value (decimal, hex, binary, char, or a label)
• ; starts a comment to end of line

Labels are resolved during pass 2, so forward references (e.g. JMP done before the done: line) work.

ByteTran

ByteTran is a Fortran-flavored high level language that compiles to BDP-1 assembly. It is uppercase, integer-only & whitespace-sensitive.

The ByteTran tab has a source editor on top and a read-only view of the generated assembly underneath that updates on every keystroke.

This shows how high level language constructs translate into assembly instructions.

Click COMPILE & LOAD to assemble that output and run it.

Grammar

program     = { statement }
statement   = IDENT '=' expr             ! assignment
            | 'WRITE' expr
            | 'DO' 'WHILE' expr { statement } 'END'
            | 'IF' expr { statement } [ 'ELSE' { statement } ] 'END'
expr        = 'READ' | additive [ ('>='|'==') additive ]
additive    = primary [ ('+'|'-') primary ]
primary     = IDENT | NUMBER | 'TRUE' | 'FALSE'
comment     = '!' to end of line

Example

! Fibonacci - read N, print fib(N)
N = READ
A = 0
B = 1
DO WHILE N >= 1
  T = A + B
  A = B
  B = T
  N = N - 1
END
WRITE A

Limitations

• Expressions do not nest: a + b + c is illegal. At most one additive operator per expression, optionally wrapped by one comparison (so a - b >= c is allowed).
• All arithmetic is 8-bit and wraps modulo 256.