Captain's Phlog                     2020.02.07
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Okay so what follows is my first attempt at
re-learning C. I studied it in college for one
month back in 1989 and promptly forgot it all.
I'm having a blest!
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/* An variation on the Little Man Computer
 * as created by D. Stuart Madnick in 1965. */

/* My version operates on 4K of hex mailboxes
 * also allowing for more instructions (A-F).
 * I have also added OUT4 whcih outputs the 
 * calculator value as an ASCII character.
 *
 * YET TO IMPLEMENT: INP */


#include <stdio.h>

/**********************
 **********************
 ** PROGRAM EXAMPLES **
 **********************
 **********************/

// Hello World
int program[] = { 0x5005,	// 0: LDA [5] ("H")
		0x9004,		// 1: OUT4 (ASCII)
		0x5006,		// 2: LDA [6] ("i")
		0x9004,		// 3: OUT4 (ASCII)
		0x0000,		// 4: HLT
		0x0048,		// 5: DAT "H"
		0x0069 };	// 6: DAT "i"

// Countdown
// int program[] = { 0x5006,	// 0: LDA [6] (0xA)
//		0x9002,		// 1: OUT2 (INT)
//		0x2007,		// 2: SUB [7] (0x1)
//		0x7005,		// 3: BRZ 5
//		0x6001,		// 4: JMP 1
//		0x0,		// 5: HLT
//		0x000A,		// 6: DAT 0xA
//		0x0001 };	// 7: DAT 0x1


// VARIABLES
  int pc = 0x0;
  int ir = 0x0;
  int opcode = 0xFFFF;
  int mailbox = 0x0;
  int calculator = 0x41;


void main() {
  printf("\nThe Little Man Computer\n   - Emulator in C -\n\n");

  do {
    ir = program[pc];	// load instruction into IR
    opcode = ir>>12;	// tease out the Opcode
    mailbox = ir - ( opcode<<12 );	// tease out the Mailbox

    switch (opcode) {
      case 1:	// ADD
        calculator = calculator + program[mailbox];
        pc++;
        break;
      case 2:	// SUB
        calculator = calculator - program[mailbox];
        pc++;
        break;
      case 3:	// STA
        program[mailbox] = calculator;
        pc++;
        break;
      case 4:	// UNDEFINED
        break;
      case 5:	// LDA
        calculator = program[mailbox];
        pc++;
        break;
      case 6:	// JMP (BRA Unconditional)
        pc = mailbox;
        break;
      case 7:	// BRZ (BRA if Zero)
        if (calculator == 0) pc = mailbox;
          else pc++;
        break;
      case 8:	// BRP (BRA if >= Zero)
        if (calculator >= 0) pc = mailbox;
          else pc++;
        break;
      case 9:	// I/O
        if (mailbox == 0x2) {		// Output Numeric
          printf("%X", calculator); }
        else if (mailbox == 0x4) {	// Output Equivalent Char
          printf("%c", calculator); }
        pc++;
        break;
      default:	// Expansion for Opcodes 0xA - 0xF
        break; }

  } while (opcode != 0x0  /* Coffee Break */ );

    printf("\n\nCalculator: %X\n\n", calculator);	// exit
}