From b93575bbcc0513814246590e121954fcea4904a9 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sun, 27 Jun 2021 22:33:20 -0400
Subject: [PATCH] Spots that b0 and b2 of 0xb4 are 'dividers', not enables.

---
 Machines/Enterprise/Dave.cpp       | 15 ++++++++++++---
 Machines/Enterprise/Dave.hpp       |  6 +++++-
 Machines/Enterprise/Enterprise.cpp |  2 +-
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/Machines/Enterprise/Dave.cpp b/Machines/Enterprise/Dave.cpp
index 513f0fcbc..7a1bec51e 100644
--- a/Machines/Enterprise/Dave.cpp
+++ b/Machines/Enterprise/Dave.cpp
@@ -234,11 +234,14 @@ void TimedInterruptSource::run_for(Cycles cycles) {
 	// Update the programmable-frequency interrupt.
 	programmable_offset_ -= cycles.as<int>();
 	if(programmable_offset_ < 0) {
-		interrupts_ |= uint8_t(Interrupt::VariableFrequency);
+		if(programmable_level_) {
+			interrupts_ |= uint8_t(Interrupt::VariableFrequency);
+		}
+		programmable_level_ ^= true;
 
 		switch(rate_) {
-			case InterruptRate::OnekHz:			programmable_offset_ = 249;					break;
-			case InterruptRate::FiftyHz:		programmable_offset_ = 4999;				break;
+			case InterruptRate::OnekHz:			programmable_offset_ = 124;					break;
+			case InterruptRate::FiftyHz:		programmable_offset_ = 2499;				break;
 			case InterruptRate::ToneGenerator0: programmable_offset_ = channels_[0].value;	break;
 			case InterruptRate::ToneGenerator1: programmable_offset_ = channels_[1].value;	break;
 		}
@@ -250,3 +253,9 @@ Cycles TimedInterruptSource::get_next_sequence_point() const {
 	// generate activity when it underflows, not when it hits zero.
 	return (programmable_offset_+1) < one_hz_offset_.as<int>() ? Cycles(programmable_offset_+1) : one_hz_offset_;
 }
+
+uint8_t TimedInterruptSource::get_divider_state() {
+	return
+		(one_hz_offset_ < half_clock_rate ? 0x4 : 0x0) |
+		(programmable_level_ ? 0x1 : 0x0);
+}
diff --git a/Machines/Enterprise/Dave.hpp b/Machines/Enterprise/Dave.hpp
index f9fa19116..c2a0873d5 100644
--- a/Machines/Enterprise/Dave.hpp
+++ b/Machines/Enterprise/Dave.hpp
@@ -122,6 +122,7 @@ class TimedInterruptSource {
 		void write(uint16_t address, uint8_t value);
 
 		uint8_t get_new_interrupts();
+		uint8_t get_divider_state();
 
 		void run_for(Cycles);
 
@@ -131,9 +132,12 @@ class TimedInterruptSource {
 		uint8_t interrupts_ = 0;
 
 		static constexpr Cycles clock_rate{250000};
+		static constexpr Cycles half_clock_rate{125000};
 
 		Cycles one_hz_offset_ = clock_rate;
-		uint16_t programmable_offset_ = 0;
+
+		int programmable_offset_ = 0;
+		bool programmable_level_ = false;
 
 		enum class InterruptRate {
 			OnekHz,
diff --git a/Machines/Enterprise/Enterprise.cpp b/Machines/Enterprise/Enterprise.cpp
index 48019ec10..87ecbe20a 100644
--- a/Machines/Enterprise/Enterprise.cpp
+++ b/Machines/Enterprise/Enterprise.cpp
@@ -342,7 +342,7 @@ template <bool has_disk_controller> class ConcreteMachine:
 						case 0xb3:	*cycle.value = pages_[3];	break;
 
 						case 0xb4:
-							*cycle.value = interrupt_mask_ | interrupt_state_;
+							*cycle.value = (interrupt_mask_ & 0xf0) | dave_timer_->get_divider_state() | interrupt_state_;
 						break;
 						case 0xb5:
 							if(active_key_line_ < key_lines_.size()) {