From acfd4dde36493c88f64cb6305543eea21d60ddbd Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Thu, 22 Jun 2017 22:44:06 -0400
Subject: [PATCH] Reduced port writes which can adjust programmatic sync, and
 prevented anything while NMI generation is active. Moved line counter
 increment from triggered by interrupt acknowledge to triggered by horizontal
 sync. In both cases, cribbing from my own earlier work. Initial results
 suggest that sync issues are resolved in third-party software.

---
 Machines/ZX8081/ZX8081.cpp | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/Machines/ZX8081/ZX8081.cpp b/Machines/ZX8081/ZX8081.cpp
index 3f772c133..8d308c5b5 100644
--- a/Machines/ZX8081/ZX8081.cpp
+++ b/Machines/ZX8081/ZX8081.cpp
@@ -35,6 +35,7 @@ int Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
 	if(previous_counter < vsync_start_cycle_ && horizontal_counter_ >= vsync_start_cycle_) {
 		video_->run_for_cycles(vsync_start_cycle_ - previous_counter);
 		set_hsync(true);
+		line_counter_ = (line_counter_ + 1) & 7;
 		if(nmi_is_enabled_) {
 			set_non_maskable_interrupt_line(true);
 		}
@@ -66,17 +67,20 @@ int Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
 	bool is_opcode_read = false;
 	switch(cycle.operation) {
 		case CPU::Z80::PartialMachineCycle::Output:
-			set_vsync(false);
-			line_counter_ = 0;
-
 			if(!(address & 2)) nmi_is_enabled_ = false;
 			if(!(address & 1)) nmi_is_enabled_ = is_zx81_;
+			if((address&3) == 3) {
+				if(!nmi_is_enabled_) {
+					set_vsync(false);
+					line_counter_ = 0;
+				}
+			}
 		break;
 
 		case CPU::Z80::PartialMachineCycle::Input: {
 			uint8_t value = 0xff;
 			if(!(address&1)) {
-				set_vsync(true);
+				if(!nmi_is_enabled_) set_vsync(true);
 
 				uint16_t mask = 0x100;
 				for(int c = 0; c < 8; c++) {
@@ -90,7 +94,6 @@ int Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
 		} break;
 
 		case CPU::Z80::PartialMachineCycle::Interrupt:
-			line_counter_ = (line_counter_ + 1) & 7;
 			*cycle.value = 0xff;
 			horizontal_counter_ = 0;
 		break;