refactoring code to simplify it

src/6502.c

@@ -1,5 +1,3 @@
-#include <stdio.h>
-
 #include "inc/types.h"
 #include "inc/memory.h"
 #include "inc/opcodes.h"
@@ -10,20 +8,24 @@ oc2 opcode_decoded_2;
 void init()
 {
     // pc is set using 0xFFFC-0xFFFD
-    mem.mar = 0xFFFD;
+    
-    mem_read();
+    dw word;
-    pc = mem.mdr << 8;
+    db low_byte;
-    mem.mar = 0xFFFC;
+    db high_byte;
-    mem_read();
+    
-    pc = mem.mdr + pc;
+    low_byte  = read_memory(0xFFFD);
+    high_byte = read_memory(0xFFFC);
+    
+    word = high_byte << 8;  // shifts high byte to its place
+    word = word | low_byte; // appends low byte to form the complete word
+    
+    pc = word;
 }
 
 void fetch()
 {
-    mem.mar = pc;
+    ir = read_memory(pc);
-    mem_read();
+    pc = pc + 1;
-    ir = mem.mdr;
-    pc++;
 }
 
 void decode()

src/inc/memory.h

@@ -1,11 +1,9 @@
-#ifndef _APPLE_I_MEMORY_LOGIC_H_
+#ifndef _APPLE_I_MEMORY_H_
-#define _APPLE_I_MEMORY_LOGIC_H_
+#define _APPLE_I_MEMORY_H_
 
 #include "types.h"
 
-mr mem; // memory "glue" logic
+  db read_memory(dw address);
-
+void write_memory(dw address, db data);
-void mem_read();
-void mem_write();
 
 #endif

src/inc/ram.h

@@ -1,11 +0,0 @@
-#ifndef _APPLE_I_RAM_H_
-#define _APPLE_I_RAM_H_
-
-#include "types.h"
-
-mr ram; // random access memory
-
-void ram_read();
-void ram_write();
-
-#endif

src/inc/rom.h

@@ -3,8 +3,39 @@
 
 #include "types.h"
 
-mr rom; // read only memory
+db rom_memory[] = {
-
+    0xD8, 0x58, 0xA0, 0x7F, 0x8C, 0x12, 0xD0, 0xA9,
-void rom_read();
+    0xA7, 0x8D, 0x11, 0xD0, 0x8D, 0x13, 0xD0, 0xC9,
+    0xDF, 0xF0, 0x13, 0xC9, 0x9B, 0xF0, 0x03, 0xC8,
+    0x10, 0x0F, 0xA9, 0xDC, 0x20, 0xEF, 0xFF, 0xA9,
+    0x8D, 0x20, 0xEF, 0xFF, 0xA0, 0x01, 0x88, 0x30,
+    0xF6, 0xAD, 0x11, 0xD0, 0x10, 0xFB, 0xAD, 0x10,
+    0xD0, 0x99, 0x00, 0x02, 0x20, 0xEF, 0xFF, 0xC9,
+    0x8D, 0xD0, 0xD4, 0xA0, 0xFF, 0xA9, 0x00, 0xAA,
+    0x0A, 0x85, 0x2B, 0xC8, 0xB9, 0x00, 0x02, 0xC9,
+    0x8D, 0xF0, 0xD4, 0xC9, 0xAE, 0x90, 0xF4, 0xF0,
+    0xF0, 0xC9, 0xBA, 0xF0, 0xEB, 0xC9, 0xD2, 0xF0,
+    0x3B, 0x86, 0x28, 0x86, 0x29, 0x84, 0x2A, 0xB9,
+    0x00, 0x02, 0x49, 0xB0, 0xC9, 0x0A, 0x90, 0x06,
+    0x69, 0x88, 0xC9, 0xFA, 0x90, 0x11, 0x0A, 0x0A,
+    0x0A, 0x0A, 0xA2, 0x04, 0x0A, 0x26, 0x28, 0x26,
+    0x29, 0xCA, 0xD0, 0xF8, 0xC8, 0xD0, 0xE0, 0xC4,
+    0x2A, 0xF0, 0x97, 0x24, 0x2B, 0x50, 0x10, 0xA5,
+    0x28, 0x81, 0x26, 0xE6, 0x26, 0xD0, 0xB5, 0xE6,
+    0x27, 0x4C, 0x44, 0xFF, 0x6C, 0x24, 0x00, 0x30,
+    0x2B, 0xA2, 0x02, 0xB5, 0x27, 0x95, 0x25, 0x95,
+    0x23, 0xCA, 0xD0, 0xF7, 0xD0, 0x14, 0xA9, 0x8D,
+    0x20, 0xEF, 0xFF, 0xA5, 0x25, 0x20, 0xDC, 0xFF,
+    0xA5, 0x24, 0x20, 0xDC, 0xFF, 0xA9, 0xBA, 0x20,
+    0xEF, 0xFF, 0xA9, 0xA0, 0x20, 0xEF, 0xFF, 0xA1,
+    0x24, 0x20, 0xDC, 0xFF, 0x86, 0x2B, 0xA5, 0x24,
+    0xC5, 0x28, 0xA5, 0x25, 0xE5, 0x29, 0xB0, 0xC1,
+    0xE6, 0x24, 0xD0, 0x02, 0xE6, 0x25, 0xA5, 0x24,
+    0x29, 0x07, 0x10, 0xC8, 0x48, 0x4A, 0x4A, 0x4A,
+    0x4A, 0x20, 0xE5, 0xFF, 0x68, 0x29, 0x0F, 0x09,
+    0xB0, 0xC9, 0xBA, 0x90, 0x02, 0x69, 0x06, 0x2C,
+    0x12, 0xD0, 0x30, 0xFB, 0x8D, 0x12, 0xD0, 0x60,
+    0x00, 0x00, 0x00, 0x0F, 0x00, 0xFF, 0x00, 0x00
+};
 
 #endif

src/inc/types.h

@@ -1,16 +1,8 @@
 #ifndef _APPLE_I_TYPES_H_
 #define _APPLE_I_TYPES_H_
 
-typedef unsigned char  db;
+typedef unsigned char  db; // data byte   (1 byte)
-typedef unsigned short dw;
+typedef unsigned short dw; // data word   (2 bytes)
-typedef unsigned int   dd;
+typedef unsigned int   dd; // data double (4 bytes)
-
-struct memory_registers
-{
-    dw mar; // memory address register
-    db mdr; // memory data register
-};
-
-typedef struct memory_registers mr;
 
 #endif

src/main.c

@@ -1,11 +1,12 @@
 #include <stdio.h>
-
 #include "inc/6502.h"
 
 int main(int argc, char **argv)
 {
     printf("Apple-I Computer\n");
-    init();
+    
-    run();
+    init(); // start the processor
+    run();  // and runs it
+    
     return 0;
 }

src/memory.c

@@ -1,9 +1,10 @@
 #include "inc/types.h"
-#include "inc/ram.h"
 #include "inc/rom.h"
 #include "inc/memory.h"
 
 /*
+    
+ memory model:
 
  + - - - - +
  | 0000    |
@@ -30,36 +31,36 @@
  + - - - - +
 
  */
+  
+db ram_memory[4096]; // total memory: 4KB
 
-void mem_read()
+db read_memory(dw address)
 {
-    if (mem.mar >= 0x0000 && mem.mar <= 0x0FFF)
+    if (address >= 0x0000 && address <= 0x0FFF)
     {
         // 4KB memory RAM
-        ram.mar = mem.mar;
+        return ram_memory[address];
-        ram_read();
-        mem.mdr = ram.mdr;
     }
-    else if (mem.mar >= 0xFF00 && mem.mar <= 0xFFFF)
+    else if (address >= 0xFF00 && address <= 0xFFFF)
     {
         // wozmon ROM
-        rom.mar = mem.mar & 0x00FF;
+        address = address & 0x00FF;
-        rom_read();
+        return rom_memory[address];
-        mem.mdr = rom.mdr;
     }
     else
     {
-        mem.mdr = 0x00;
+        // unused memory
+        return 0x00;
     }
 }
 
-void mem_write()
+void write_memory(dw address, db data)
 {
-    if (mem.mar >= 0x0000 && mem.mar <= 0x0FFF)
+    if (address >= 0x0000 && address <= 0x0FFF)
     {
         // 4KB memory RAM
-        ram.mar = mem.mar;
+        ram_memory[address] = data;
-        ram.mdr = mem.mdr;
-        ram_write();
     }
+    
+    // any other addressed memory will be ignored on write
 }

src/opcodes.c

@@ -3,23 +3,16 @@
 #include "inc/opcodes.h"
 #include "inc/memory.h"
 
-db fetch_operand(dw address)
-{
-    mem.mar = address;
-    mem_read();
-    return mem.mdr;
-}
-
 dw fetch_address()
 {
     dw word;
     db low_byte;
     db high_byte;
     
-    low_byte = fetch_operand(pc);
+    low_byte = read_memory(pc);
     pc = pc + 1;
     
-    high_byte = fetch_operand(pc);
+    high_byte = read_memory(pc);
     pc = pc + 1;
     
     word = high_byte << 8;  // shifts high byte to its place
@@ -37,27 +30,27 @@ db decode_operand()
     switch (addressing_mode)
     {
         case immediate:
-            operand = fetch_operand(pc);
+            operand = read_memory(pc);
             pc = pc + 1;
             break;
         case zero_page:
-            byte = fetch_operand(pc);
+            byte = read_memory(pc);
             word = 0x0000 | byte;
-            operand = fetch_operand(word);
+            operand = read_memory(word);
             pc = pc + 1;
             break;
         case zero_page_x:
-            byte = fetch_operand(pc);
+            byte = read_memory(pc);
             word = 0x0000 | byte;
             word = word + x;
-            operand = fetch_operand(word);
+            operand = read_memory(word);
             pc = pc + 1;
             break;
         case zero_page_y:
-            byte = fetch_operand(pc);
+            byte = read_memory(pc);
             word = 0x0000 | byte;
             word = word + y;
-            operand = fetch_operand(word);
+            operand = read_memory(word);
             pc = pc + 1;
             break;
         case accumulator:

src/ram.c

@@ -1,14 +0,0 @@
-#include "inc/types.h"
-#include "inc/ram.h"
-
-db ram_memory[4096]; // total memory: 4KB
-
-void ram_read()
-{
-    ram.mdr = ram_memory[ram.mar];
-}
-
-void ram_write()
-{
-    ram_memory[ram.mar] = ram.mdr;
-}

src/rom.c

@@ -1,42 +0,0 @@
-#include "inc/types.h"
-#include "inc/rom.h"
-
-db rom_memory[] = {
-    0xD8, 0x58, 0xA0, 0x7F, 0x8C, 0x12, 0xD0, 0xA9,
-    0xA7, 0x8D, 0x11, 0xD0, 0x8D, 0x13, 0xD0, 0xC9,
-    0xDF, 0xF0, 0x13, 0xC9, 0x9B, 0xF0, 0x03, 0xC8,
-    0x10, 0x0F, 0xA9, 0xDC, 0x20, 0xEF, 0xFF, 0xA9,
-    0x8D, 0x20, 0xEF, 0xFF, 0xA0, 0x01, 0x88, 0x30,
-    0xF6, 0xAD, 0x11, 0xD0, 0x10, 0xFB, 0xAD, 0x10,
-    0xD0, 0x99, 0x00, 0x02, 0x20, 0xEF, 0xFF, 0xC9,
-    0x8D, 0xD0, 0xD4, 0xA0, 0xFF, 0xA9, 0x00, 0xAA,
-    0x0A, 0x85, 0x2B, 0xC8, 0xB9, 0x00, 0x02, 0xC9,
-    0x8D, 0xF0, 0xD4, 0xC9, 0xAE, 0x90, 0xF4, 0xF0,
-    0xF0, 0xC9, 0xBA, 0xF0, 0xEB, 0xC9, 0xD2, 0xF0,
-    0x3B, 0x86, 0x28, 0x86, 0x29, 0x84, 0x2A, 0xB9,
-    0x00, 0x02, 0x49, 0xB0, 0xC9, 0x0A, 0x90, 0x06,
-    0x69, 0x88, 0xC9, 0xFA, 0x90, 0x11, 0x0A, 0x0A,
-    0x0A, 0x0A, 0xA2, 0x04, 0x0A, 0x26, 0x28, 0x26,
-    0x29, 0xCA, 0xD0, 0xF8, 0xC8, 0xD0, 0xE0, 0xC4,
-    0x2A, 0xF0, 0x97, 0x24, 0x2B, 0x50, 0x10, 0xA5,
-    0x28, 0x81, 0x26, 0xE6, 0x26, 0xD0, 0xB5, 0xE6,
-    0x27, 0x4C, 0x44, 0xFF, 0x6C, 0x24, 0x00, 0x30,
-    0x2B, 0xA2, 0x02, 0xB5, 0x27, 0x95, 0x25, 0x95,
-    0x23, 0xCA, 0xD0, 0xF7, 0xD0, 0x14, 0xA9, 0x8D,
-    0x20, 0xEF, 0xFF, 0xA5, 0x25, 0x20, 0xDC, 0xFF,
-    0xA5, 0x24, 0x20, 0xDC, 0xFF, 0xA9, 0xBA, 0x20,
-    0xEF, 0xFF, 0xA9, 0xA0, 0x20, 0xEF, 0xFF, 0xA1,
-    0x24, 0x20, 0xDC, 0xFF, 0x86, 0x2B, 0xA5, 0x24,
-    0xC5, 0x28, 0xA5, 0x25, 0xE5, 0x29, 0xB0, 0xC1,
-    0xE6, 0x24, 0xD0, 0x02, 0xE6, 0x25, 0xA5, 0x24,
-    0x29, 0x07, 0x10, 0xC8, 0x48, 0x4A, 0x4A, 0x4A,
-    0x4A, 0x20, 0xE5, 0xFF, 0x68, 0x29, 0x0F, 0x09,
-    0xB0, 0xC9, 0xBA, 0x90, 0x02, 0x69, 0x06, 0x2C,
-    0x12, 0xD0, 0x30, 0xFB, 0x8D, 0x12, 0xD0, 0x60,
-    0x00, 0x00, 0x00, 0x0F, 0x00, 0xFF, 0x00, 0x00
-};
-
-void rom_read()
-{
-    rom.mdr = rom_memory[rom.mar];
-}