documenting VM hacks

issue #17
felipesanches · Jun 1, 2017 · 889934f · 889934f
1 parent f1a0960
commit 889934f
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diff --git a/src/devices/cpu/anotherworld/anotherworld.cpp b/src/devices/cpu/anotherworld/anotherworld.cpp
@@ -232,7 +232,22 @@ void another_world_cpu_device::device_reset()
         thread->requested_state = NO_REQUEST;
     }
 
-    write_vm_variable(0x54, 0x0081); //TODO: figure out why this is supposedly needed.
+#ifdef VM_HACK_INIT_VAR_54_WITH_81
+    // Without this, the Interplay logo does not show up
+    // while running the MSDOS bytecode:
+    write_vm_variable(0x54, 0x0081);
+
+    //TODO: check if this var is read or written anywhere
+    // else during gameplay. Ideally run the whole game with
+    // a watchpoint monitoring this one.
+
+// Some clues:
+// [14 - 007F]: 0A jl [HACK_VAR_54], 0x80, 0x00BF
+// [14 - 00E4]: 0A jge [HACK_VAR_54], 0x80, 0x00F6
+// [14 - 0165]: 14 and [HACK_VAR_54], 0x0001
+// [14 - 0169]: 0A je [HACK_VAR_54], 0x00, 0x0199
+// [18 - 3EAB]: 00 mov [HACK_VAR_54], 0xFFFB
+#endif
 
 #ifdef BYPASS_PROTECTION 
     //This is a hack to skip the code wheel
@@ -572,12 +587,29 @@ void another_world_cpu_device::execute_instruction()
         {
             uint8_t pageId = fetch_byte();
             //printf("blitFramebuffer(pageId:%d, PAUSE_SLICES:%d)\n", pageId, read_vm_variable(VM_VARIABLE_PAUSE_SLICES));
-#if 1
+
+#ifdef VM_HACK_SWITCH_FROM_INTRO_TO_LAKE
             //Nasty hack....was this present in the original assembly  ??!!
             //This seems to be necessary for switching from the intro sequence to the lake stage
             if (m_currentPartId == GAME_PART(0) && read_vm_variable(0x67) == 1)
                 write_vm_variable(0xDC, 0x21);
+
+// Some clues:
+// [32 - 0D41]: 00 mov [HACK_VAR_67], 0x0001
+// [3F - 0D46]: 00 mov [HACK_VAR_67], 0x0000
+// [00 - 00B2]: 0A jne [HACK_VAR_DC], 0x21, 0x00B9
+// [03 - 00B2]: 0A jne [HACK_VAR_DC], 0x21, 0x00B9
+// [3F - 19BB]: 00 mov [HACK_VAR_67], 0x0002
+// [3F - 1954]: 0A jne [HACK_VAR_67], [0x65], 0x196F
+// [2E - 35E2]: 0A jne [HACK_VAR_67], 0x01, 0x35E1
+// [30 - 01F0]: 0A je [HACK_VAR_67], 0x05, 0x01E9
+// [30 - 01F6]: 0A je [HACK_VAR_67], 0x02, 0x01E9
+// [30 - 0226]: 0A je [HACK_VAR_67], 0x05, 0x0238
+
+// Much more with:
+// grep HACK_VAR_67 address_log.txt|sort|uniq|less
 #endif
+
             // The bytecode will set vmVariables[VM_VARIABLE_PAUSE_SLICES] from 1 to 5
             // The virtual machine hence indicate how long the image should be displayed.
 
@@ -587,9 +619,13 @@ void another_world_cpu_device::execute_instruction()
             m_icount -= (int) read_vm_variable(VM_VARIABLE_PAUSE_SLICES);
 #endif
 
+#ifdef VM_HACK_BLITFRAMEBUFFER_VAR_F7
             //Why ?
             write_vm_variable(0xF7, 0x0000);
 
+// Some clues:
+// [14 - 0A1B]: 02 add [0x37], [HACK_VAR_F7]
+#endif
             ((another_world_state*) owner())->updateDisplay(pageId);
             return;
         }

diff --git a/src/devices/cpu/anotherworld/anotherworld.h b/src/devices/cpu/anotherworld/anotherworld.h
@@ -5,6 +5,16 @@
 #ifndef __ANOTHERWORLD_H__
 #define __ANOTHERWORLD_H__
 
+//VM irregularities (see: https://github.com/felipesanches/mame/issues/17)
+#define VM_HACK_SWITCH_FROM_INTRO_TO_LAKE // Seems needed both on Amiga and MSDOS bytecode.
+#define VM_HACK_BLITFRAMEBUFFER_VAR_F7    // Seems to be unnecessary.
+#define VM_HACK_INIT_VAR_54_WITH_81       // Without this one, the Interplay logo does not show up with MSDOS bytecode
+
+//a few optional features for easing VM debugging:
+#define DUMP_VM_EXECUTION_LOG
+//#define SPEEDUP_VM_EXECUTION
+//#define BYPASS_PROTECTION
+
 #include <fstream>
 #include <iostream>
 
@@ -39,11 +49,6 @@ enum {
 #define NUM_THREADS 64
 #define GAME_PART(n) (0x3E80 + n)
 
-//a few optional features for easing VM debugging:
-#define DUMP_VM_EXECUTION_LOG
-//#define SPEEDUP_VM_EXECUTION
-//#define BYPASS_PROTECTION
-
 #define AS_PROGRAM AS_0
 #define AS_DATA AS_1
 #define AS_PALETTE AS_2
@@ -52,9 +57,13 @@ enum {
 
 enum ScriptVars {
     VM_VARIABLE_RANDOM_SEED          = 0x3C,
+    VM_VARIABLE_HACK_VAR_54          = 0x54, // Interplay logo  TODO: verify this
+    VM_VARIABLE_HACK_VAR_67          = 0x67, // switch from intro to lake  TODO: verify this
     VM_VARIABLE_LAST_KEYCHAR         = 0xDA,
+    VM_VARIABLE_HACK_VAR_DC          = 0xDC, // switch from intro to lake  TODO: verify this
     VM_VARIABLE_HERO_POS_UP_DOWN     = 0xE5,
     VM_VARIABLE_MUS_MARK             = 0xF4,
+    VM_VARIABLE_HACK_VAR_F7          = 0xF7, // Blitframebuffer (not needed?)  TODO: verify this
     VM_VARIABLE_SCROLL_Y             = 0xF9,
     VM_VARIABLE_HERO_ACTION          = 0xFA,
     VM_VARIABLE_HERO_POS_JUMP_DOWN   = 0xFB,

diff --git a/src/devices/cpu/anotherworld/anotherworld_dasm.cpp b/src/devices/cpu/anotherworld/anotherworld_dasm.cpp
@@ -18,6 +18,10 @@ static void getVariableName(char* s, uint8_t id){
         case VM_VARIABLE_HERO_POS_MASK:         sprintf(s, "HERO_POS_MASK"); break;
         case VM_VARIABLE_HERO_ACTION_POS_MASK:  sprintf(s, "HERO_ACTION_POS_MASK"); break;
         case VM_VARIABLE_PAUSE_SLICES:          sprintf(s, "PAUSE_SLICES"); break;
+        case VM_VARIABLE_HACK_VAR_54:           sprintf(s, "HACK_VAR_54"); break;
+        case VM_VARIABLE_HACK_VAR_67:           sprintf(s, "HACK_VAR_67"); break;
+        case VM_VARIABLE_HACK_VAR_DC:           sprintf(s, "HACK_VAR_DC"); break;
+        case VM_VARIABLE_HACK_VAR_F7:           sprintf(s, "HACK_VAR_F7"); break;
         default:
             sprintf(s, "0x%02X", id); break;
     }