From fea32869b0d5aee72019dc7950d51dd6dde19391 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Thu, 5 May 2016 08:28:06 -0400
Subject: [PATCH] Implemented what I now believe to be the real paging logic.

---
 Machines/Electron/Electron.cpp | 54 ++++++++++++++++------------------
 Machines/Electron/Electron.hpp |  1 +
 2 files changed, 27 insertions(+), 28 deletions(-)

diff --git a/Machines/Electron/Electron.cpp b/Machines/Electron/Electron.cpp
index e2453dfd2..c6b90b1a9 100644
--- a/Machines/Electron/Electron.cpp
+++ b/Machines/Electron/Electron.cpp
@@ -175,26 +175,25 @@ unsigned int Machine::perform_bus_operation(CPU6502::BusOperation operation, uin
 								if( interruptDisable&0x40 ) _interrupt_status &= ~Interrupt::HighToneDetect;
 								evaluate_interrupts();
 
-								// TODO: NMI (?)
+								// TODO: NMI
 							}
-//							else
-							{
-								uint8_t nextROM = (*value)&0xf;
 
-//								if(nextROM&0x08)
-//								{
-//									_activeRom = (Electron::ROMSlot)(nextROM&0x0e);
-//									printf("%d -> Paged %d\n", nextROM, _activeRom);
-//								}
-								if(((_active_rom&12) != 8) || (nextROM&8))
+							// latch the paged ROM in case external hardware is being emulated
+							_active_rom = (Electron::ROMSlot)(*value & 0xf);
+
+							// apply the ULA's test
+							if(*value & 0x08)
+							{
+								if(*value & 0x04)
 								{
-									_active_rom = (Electron::ROMSlot)nextROM;
+									_keyboard_is_active = false;
+									_basic_is_active = false;
+								}
+								else
+								{
+									_keyboard_is_active = !(*value & 0x02);
+									_basic_is_active = !_keyboard_is_active;
 								}
-//								else
-//								{
-//									printf("Ignored!");
-//								}
-//								printf("%d -> Paged %d\n", nextROM, _activeRom);
 							}
 						}
 					break;
@@ -375,19 +374,18 @@ unsigned int Machine::perform_bus_operation(CPU6502::BusOperation operation, uin
 		{
 			if(isReadOperation(operation))
 			{
-				switch(_active_rom)
+				*value = _roms[_active_rom][address & 16383];
+				if(_keyboard_is_active)
 				{
-					case ROMSlotKeyboard:
-					case ROMSlotKeyboard+1:
-						*value = 0xf0;
-						for(int address_line = 0; address_line < 14; address_line++)
-						{
-							if(!(address&(1 << address_line))) *value |= _key_states[address_line];
-						}
-					break;
-					default:
-						*value = _roms[_active_rom][address & 16383];
-					break;
+					*value &= 0xf0;
+					for(int address_line = 0; address_line < 14; address_line++)
+					{
+						if(!(address&(1 << address_line))) *value |= _key_states[address_line];
+					}
+				}
+				if(_basic_is_active)
+				{
+					*value &= _roms[ROMSlotBASIC][address & 16383];
 				}
 			}
 		}
diff --git a/Machines/Electron/Electron.hpp b/Machines/Electron/Electron.hpp
index 791256d9a..151cdbe54 100644
--- a/Machines/Electron/Electron.hpp
+++ b/Machines/Electron/Electron.hpp
@@ -187,6 +187,7 @@ class Machine: public CPU6502::Processor<Machine>, Tape::Delegate {
 		uint8_t _palette[16];
 		uint8_t _key_states[14];
 		ROMSlot _active_rom;
+		bool _keyboard_is_active, _basic_is_active;
 		uint8_t _screen_mode;
 		uint16_t _screenModeBaseAddress;
 		uint16_t _startScreenAddress;