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LM-8 Architecture

The architectural design of this computer was heavily influenced by classic 8-bit processors like the 6502.

Specifications

  • 8-bit (Obviously)
  • 16-bit big-endian addressing
  • Only 27 unique instructions
  • 4 8-bit general program registers
  • 4-bit status register
  • Programmable CPU microcode
  • 32 KB of both ROM and RAM
  • 160x128 display output with sprite rendering and double buffering

Memory Map

  • 0x0000 - 0x7FFF: Program ROM
  • 0x8000 - 0xFEFF: Program RAM
  • 0xFF00 - 0xFFFF: Stack

Microcode

The microcode.py script generates the microcode.bin binary which controls the CPU. It works by specifying which wires should be high or low based on which state of which instruction is currently active in the CPU. There are 64 possible instructions, with each instruction having upto 6 states, with each state occupying 3 bytes or 24 wires.

CPU Flags

  • Z: The Zero flag, set if the result of an instruction is zero.
  • C: The Carry flag, set if there is a carry or borrow during an instruction.
  • N: The Negative flag, set if the signed result of an instruction is negative.
  • V: The Overflow flag, set if a signed overflow takes place during an instruction.

Instructions

Instruction Machine Code Function Flags (ZCNV) Cycles
DEF term=value N/A Define a program constant N/A N/A
ORG addr N/A Set the assembler origin N/A N/A
INCLUDE N/A Include an assembly file N/A N/A
DATA N/A Enter the data section N/A N/A
VAR N/A A variable placeholder N/A N/A
VAR[n] N/A A variable array placeholder N/A N/A
DB "HI",'\n',$0 N/A Define bytes N/A N/A
BIN N/A Inject a binary file N/A N/A
NOP 000000** No operation **** 1
LDR imm,reg 000001rr reg = imm Z*** 3
LDR [addr],reg 000010rr reg = [addr] Z*** 7
LDR [HL],reg 000011rr reg = [HL] Z*** 3
STR [addr],reg 000100rr [addr] = reg **** 6
STR [HL],reg 000101rr [HL] = reg **** 2
LDA addr 000110** HL = addr **** 5
IN imm,reg 000111rr reg = PORT[imm] Z*** 4
OUT imm,reg 001000rr PORT[imm] = reg **** 4+
INC reg 001001rr reg = reg + 1 ZC** 1
DEC reg 001010rr reg = reg - 1 ZC** 1
INA 001011** HL = HL + 1 **** 2 or 3
DEA 001100** HL = HL - 1 **** 2 or 3
ADD imm 001101** A = A + imm ZCNV 4
ADD reg 001110rr A = A + reg ZCNV 1
ADC imm 001111** A = A + imm + C ZCNV 4
ADC reg 010000rr A = A + reg + C ZCNV 1
SUB imm 010001** A = A - imm ZCNV 4
SUB reg 010010rr A = A - reg ZCNV 1
SBC imm 010011** A = A - imm - C ZCNV 4
SBC reg 010100rr A = A - reg - C ZCNV 1
AND imm 010101** A = A & imm Z*** 4
AND reg 010110rr A = A & reg Z*** 1
OR imm 010111** A = A | imm Z*** 4
OR reg 011000rr A = A | reg Z*** 1
XOR imm 011001** A = A ^ imm Z*** 4
XOR reg 011010rr A = A ^ reg Z*** 1
CMP imm 011011** A - imm ZCNV 4
CMP reg 011100rr A - reg ZCNV 1
JMP addr 011101** PC = addr **** 6
JMP HL 011110** PC = HL **** 1
JR rel 011111** PC = PC + rel **** 4
JR rel,cc 100000cc If cc, PC = PC + rel **** 4 or 5
JR rel,nn 100001nn If nn, PC = PC + rel **** 4 or 5
IN reg 100010rr reg = PORT[A] Z*** 3
OUT reg 100011rr PORT[A] = reg **** 3+
PUSH reg 100100rr [--SP] = reg **** 2
POP reg 100101rr reg = [SP++] **** 3
JSR addr 100110** SP = SP - 2, [SP] = PC **** 8
RET 100111** PC = [SP], SP = SP + 2 **** 6
HALT 101000** Halt CPU **** 2
LSL 101001** C <-- reg <-- 0 ZC** 1
LSR 101010** 0 --> reg --> C ZC** 1
ASR 101011** reg[7] --> reg --> C ZC** 1

Key:

  • *: Unused
  • rr: Register (A, B, H, or L)
  • cc: Condition (Z, C, N, or V)
  • nn: Condition Inverse (nZ, nC, nN, or nV)
  • imm: 8-bit Immediate (%10101010, #255, #-90, $FF, or '\n')
  • rel: Relative Distance (Label or signed 8-bit immediate)
  • addr: Address (Label or 16-bit immediate)

Cycle Exceptions

  • OUT instructions can take any number more than 4 cycles based on the port selected.
  • JR instructions with conditions take 4 if the condition failed and 5 otherwise.
  • INA and DEA instructions take an extra cycle if L rolls around and changes H.

I/O Ports

For binary ports, reading will return a 0 or 1 and writing anything but a 0 is seen as a 1.

Port Id(s) Function
serial 0 Output a character to the console or UART, read to get UART input
serial_available 1 Read to get available UART bytes, write to pop a value
seven_segment_n 2-7 Access seven segment display register n (0-5)
button_n 8-9 Read the binary state of button n (0-1)
led_n 10-19 Read or write to LED n (0-9)
switch_n 20-29 Read the binary state of switch n (0-9)
graphics_x 30 Access the graphics X register
graphics_y 31 Access the graphics Y register
draw_pixel 32 Draws a single RGB332 pixel at (X,Y)
draw_sprite 33 Draws an 8x8 RGB332 sprite from the address (HL) at (X,Y)
clear_screen 34 Clears the screen with the specified RGB332 color
gpio_n 35-70 Access GPIO pin n (0-35)
arduino_n 71-86 Access Arduino header I/O pin n (0-15)
adc_n 87-92 Read from ADC n (0-6)
rand 93 Read a random number
swap_display 94 Swap the emulator display buffers
gpio_output 95 Set the specified GPIO pin to output mode (0-35)
gpio_input 96 Set the specified GPIO pin to input mode (0-35)
arduino_output 97 Set the Arduino pin (value & 0xF) to output mode (0-15)
arduino_input 98 Set the Arduino pin (value & 0xF) to input mode (0-15)
pwm_enable 99 Enable PWM for a compatible Arduino pin (3, 5, 6, 9, 10, 11)
pwm_disable 100 Disable PWM for a compatible Arduino pin (3, 5, 6, 9, 10, 11)
pwm_n 101-106 Set the PWM duty cycle for compatible pin n (3, 5, 6, 9, 10, 11)
serial_enable 107 Enables or disables the UART output, TX, on Arduino pin 1
timer_unit_n 108-109 Sets the timer count of timer n (0-1) to (50 * 10 ^ value) (0-7)
timer_count_n 110-111 Sets the count multiplier for timer n (0-1)
timer_n 112-113 Returns whether timer n (0-1) has triggered, write to reset timer