m6502: Seriously untested multi-dispatch-table support [O. Galibert]

docs/m6502.txt

@@ -252,7 +252,7 @@ A code generator is used to support interrupting and restarting an
 instruction in the middle.  This is done through a two-level state
 machine with updates only at the boundaries.  More precisely,
 inst_state tells you which main state you're in.  It's equal to the
-opcode byte when 0-255, and 256 means reset.  It's always valid and
+opcode byte when 0-255, and 0xff00 means reset.  It's always valid and
 used by instructions like rmb.  inst_substate indicates at which step
 we are in an instruction, but it set only when an instruction has been
 interrupted.  Let's go back to the asl <abs> code:
@@ -458,7 +458,19 @@ to note that the exception path only happens when the contention/wait
 state goes further than the scheduling slice of the cpu.  That should
 not usually be the case, so the cost should be minimal.
 
-  10. Current TODO
+  10. Multi-dispatch variants
+
+Some variants currently in the process of being supported change
+instruction set depending on an internal flag, either switching to a
+16-bits mode or changing some register accesses to memory accesses.
+This is handled by having multiple dispatch tables for the cpu, the
+d<cpu>.lst not being 257 entries anymore but 256*n+1.  The variable
+inst_state_base must select which instruction table to use at a given
+time.  It must be a multiple of 256, and is in fact simply or-ed to
+the first instruction byte to get the dispatch table index (aka
+inst_state).
+
+  11. Current TODO
 
 - Implement the bus contention/wait states stuff, but that requires
   support on the memory map side first.

src/emu/cpu/m6502/m6502.c

@@ -100,6 +100,7 @@ void m6502_device::init()
 	save_item(NAME(v_state));
 	save_item(NAME(inst_state));
 	save_item(NAME(inst_substate));
+	save_item(NAME(inst_state_base));
 	save_item(NAME(irq_taken));
 	save_item(NAME(inhibit_interrupts));
 
@@ -122,6 +123,7 @@ void m6502_device::init()
 	v_state = false;
 	inst_state = STATE_RESET;
 	inst_substate = 0;
+	inst_state_base = 0;
 	sync = false;
 	end_cycles = 0;
 	inhibit_interrupts = false;
@@ -131,6 +133,7 @@ void m6502_device::device_reset()
 {
 	inst_state = STATE_RESET;
 	inst_substate = 0;
+	inst_state_base = 0;
 	nmi_state = false;
 	irq_state = false;
 	apu_irq_state = false;
@@ -406,7 +409,7 @@ void m6502_device::execute_run()
 	while(icount > 0) {
 		if(inst_state < 0x100) {
 			PPC = NPC;
-			inst_state = IR;
+			inst_state = IR | inst_state_base;
 			if(machine().debug_flags & DEBUG_FLAG_ENABLED)
 				debugger_instruction_hook(this, NPC);
 		}
@@ -479,7 +482,7 @@ UINT32 m6502_device::disasm_max_opcode_bytes() const
 
 offs_t m6502_device::disassemble_generic(char *buffer, offs_t pc, const UINT8 *oprom, const UINT8 *opram, UINT32 options, const disasm_entry *table)
 {
-	const disasm_entry &e = table[oprom[0]];
+	const disasm_entry &e = table[oprom[0] | inst_state_base];
 	UINT32 flags = e.flags | DASMFLAG_SUPPORTED;
 	buffer += sprintf(buffer, "%s", e.opcode);
 

src/emu/cpu/m6502/m6502.h

@@ -97,7 +97,7 @@ protected:
 	};
 
 	enum {
-		STATE_RESET = 0x100,
+		STATE_RESET = 0xff00,
 	};
 
 	enum {
@@ -182,6 +182,7 @@ protected:
 	UINT8   Y;                      /* Y index register */
 	UINT8   P;                      /* Processor status */
 	UINT8   IR;                     /* Prefetched instruction register */
+	int     inst_state_base;        /* Current instruction bank */
 
 	memory_interface *mintf;
 	int inst_state, inst_substate;

src/emu/cpu/m6502/m6502make.py

@@ -7,7 +7,7 @@ Usage:
 import sys
 import logging
 
-MAX_STATES = 0x101
+MAX_STATES = 0
 
 def load_opcodes(fname):
     """Load opcodes from .lst file"""
@@ -174,7 +174,7 @@ DO_EXEC_PARTIAL_EPILOG="""\
 """
 
 DISASM_PROLOG="""\
-const %(device)s::disasm_entry %(device)s::disasm_entries[0x100] = {
+const %(device)s::disasm_entry %(device)s::disasm_entries[0x%(disasm_count)x] = {
 """
 
 DISASM_EPILOG="""\
@@ -182,15 +182,17 @@ DISASM_EPILOG="""\
 """
 
 def save_tables(f, device, states):
+    total_states = len(states)
+
     d = { "device": device,
+          "disasm_count": total_states-1
           }
-
-    assert len(states) == MAX_STATES
-
+    
+    
     emit(f, DO_EXEC_FULL_PROLOG % d)
     for n, state in enumerate(states):
         if state == ".": continue
-        if n < MAX_STATES - 1:
+        if n < total_states - 1:
             emit(f, "\tcase 0x%02x: %s_full(); break;\n" % (n, state))
         else:
             emit(f, "\tcase %s: %s_full(); break;\n" % ("STATE_RESET", state))
@@ -199,16 +201,16 @@ def save_tables(f, device, states):
     emit(f, DO_EXEC_PARTIAL_PROLOG % d)
     for n, state in enumerate(states):
         if state == ".": continue
-        if n < MAX_STATES - 1:
+        if n < total_states - 1:
             emit(f, "\tcase 0x%02x: %s_partial(); break;\n" % (n, state))
         else:
             emit(f, "\tcase %s: %s_partial(); break;\n" % ("STATE_RESET", state))
     emit(f, DO_EXEC_PARTIAL_EPILOG % d)
 
-    emit(f, DISASM_PROLOG % d)
+    emit(f, DISASM_PROLOG % d )
     for n, state in enumerate(states):
         if state == ".": continue
-        if n == MAX_STATES - 1: break
+        if n == total_states - 1: break
         tokens = state.split("_")
         opc = tokens[0]
         mode = tokens[-1]
@@ -262,8 +264,8 @@ def main(argv):
 
     states = load_disp(argv[3])
     logging.info("loaded %s states", len(states))
-    assert len(states) == MAX_STATES
 
+    assert (len(states) & 0xff) == 1
     save(argv[4], device_name, opcodes, states)