From 51581e687910f75d6e9e06058ca8c2a3370ad290 Mon Sep 17 00:00:00 2001
From: n loewen <nick@nloewen.com>
Date: Tue, 25 Jul 2023 15:05:11 +0100
Subject: [PATCH] Initial proof-of-concept -- a working but incomplete
 simulator

---
 readme.md           |  52 ++++++++++++++++++
 simulator-sketch.js | 131 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 183 insertions(+)
 create mode 100644 readme.md
 create mode 100644 simulator-sketch.js

diff --git a/readme.md b/readme.md
new file mode 100644
index 0000000..067c419
--- /dev/null
+++ b/readme.md
@@ -0,0 +1,52 @@
+# Paper computer simulator experiment
+
+## ISA
+
+0 END
+1 STO lit# ; store ... mem[lit#] <- A
+2 STO addr ; store ... mem[mem[addr]] <- A
+3 LDA lit# ; load  ... A <- lit#
+4 LDA addr ; load  ... A <- mem[addr]
+5 ADD lit# ; add   ... A <- A + lit#     ... and un/set carry flag
+6 ADD addr ; add   ... A <- A + mem[addr] ... and un/set carry flag
+7 SUB lit# ; sub   ... A <- A - lit#     ... and un/set carry flag
+8 SUB addr ; sub   ... A <- A - mem[addr] ... and un/set carry flag
+9 HOP lit# ; skip next instruction if A == lit# ... when true: IP <- PC + 2
+A HOP addr ; skip next instruction if A == addr ... when true: IP <- PC + 2
+B JMP lit# ; ... IP <- lit#
+C JMP addr ; ... IP <- addr
+D CAT ---- ; Carry Flag Toggle ... CF <- !CF
+E CHP ---- ; skip next instruction if Carry Flag is set ... when true: IP <- PC + 2
+F
+
+### Nice features that didn't fit
+
+- hop IF< and hop IF>
+- MUL and DIV
+- rotates and shifts
+
+## Registers
+
+- A -- accumulator
+- IP -- instruction pointer (aka program counter)
+
+## Flags
+
+- CF -- carry flag
+
+## Memory-mapped peripherals
+
+- 4x4 display
+- hex keypad (details TBD)
+- ? bank selector (for bank-switching)
+
+### Maybe someday
+
+- timer (for a version in software/electronic-hardware)
+
+## Tentative memory map
+
+00-0F - display
+10-1F - keypad? (details TBD)
+20    - initial value for IP ?
+21-FF - free
\ No newline at end of file
diff --git a/simulator-sketch.js b/simulator-sketch.js
new file mode 100644
index 0000000..3a8b91e
--- /dev/null
+++ b/simulator-sketch.js
@@ -0,0 +1,131 @@
+function CPU(mem) {
+  this.memory = mem;
+  this.running = false;
+  this.instructionPointer = 0;
+  this.carryFlag =  0;
+  this.acc = 0;
+
+  this.instructions = {
+    end:        ()     => {
+      console.log('END');
+      this.running = false
+    },
+    store_lit:  (lit)  => {
+      console.log('STO lit#');
+      this.memory[lit] = this.acc;
+      log_memory();
+      this.instructionPointer = this.instructionPointer += 1;
+    },
+    store_addr: (addr) => {
+      console.log('STO addr');
+      this.memory[this.memory[addr]] = this.acc;
+      log_memory();
+      this.instructionPointer = this.instructionPointer += 1;
+    },
+    load_lit:   (lit)  => {
+      console.log('LDA lit#');
+      this.acc = lit;
+      this.instructionPointer = this.instructionPointer += 1;
+    },
+    load_addr:  (addr) => {
+      console.log('LDA addr');
+      console.log('mem at addr: ', this.memory[addr]);
+      this.acc = this.memory[addr];
+      this.instructionPointer = this.instructionPointer += 1;
+    },
+  };
+
+  log_debug_state = () => {
+    console.log();
+    console.group('CPU state');
+    console.log( `IP: ${this.instructionPointer}   Acc: ${this.acc}   CF: ${this.carryFlag}   ${this.running ? "running" : "halted" }` );
+    console.log();
+    console.groupEnd('CPU state');
+  };
+
+  log_memory = () => {
+    console.log();
+    console.group('Memory');
+    console.table(this.memory);
+    console.groupEnd('Memory');
+  }
+
+  this.perform_operation = (opcode, arg) => {
+    switch (opcode) {
+      case 0:
+        this.instructions.end(arg);
+        break;
+
+      case 1:
+        this.instructions.store_lit(arg);
+        break;
+
+      case 2:
+        this.instructions.store_addr(arg);
+        break;
+
+      case 3:
+        this.instructions.load_lit(arg);
+        break;
+
+      case 4:
+        this.instructions.load_addr(arg);
+        break;
+      
+      default:
+        console.error( `Invalid opcode: ${opcode} with argument ${arg}` );
+    }
+  };
+
+  this.run_program = () => {
+    console.log();
+    console.log( "Running program..." );
+    log_debug_state();
+
+    this.running = true;
+
+    for (let i = 1; i < 16; i++) {
+      if ( this.running &&
+            (this.instructionPointer < this.memory.length) ) {
+        let op_arg_tuple = this.memory[this.instructionPointer];
+        console.group("Proccessing instruction");
+        console.log( op_arg_tuple );
+        // console.log( `processing opcode ${op_arg_tuple[0]} with arg ${op_arg_tuple[1]}` );
+        this.perform_operation(op_arg_tuple[0], op_arg_tuple[1]);
+        log_debug_state();
+        console.groupEnd("Processing instruction");
+      }
+    }
+  }
+};
+
+// UNIMPLEMENTED
+  let add_lit = function(lit) { return; }
+  let add_addr = function(addr) { return; }
+  let subtract_lit = function(lit) { return; }
+  let subtract_addr = function(addr) { return; }
+  let hop_lit = function(lit) { return; }
+  let hop_addr = function(addr) { return; }
+  let jump_lit = function(lit) { return; }
+  let jump_addr = function(addr) { return; }
+  let carry_toggle = function() { return; }
+  let carry_hop = function() { return; }
+
+
+// TEST
+
+let halt_and_catch_fire = [
+  [0, 0],
+  [1, 0],
+];
+
+let basic_test = [
+  [3, 8], // LDA lit
+  [1, 5], // STO lit
+  [4, 5], // LDA addr
+  [2, 6], // STO addr
+  [0, 0], // END
+];
+
+let comp = new CPU(basic_test);
+comp.run_program();
\ No newline at end of file