From fe07d8c3b7248d5affce93beded75ca24837efe7 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Thu, 9 Apr 2026 10:12:01 -0400
Subject: [PATCH] Attempt further to discern MO6 meaning.

---
 Machines/Thomson/MO5/MemoryMap.hpp | 66 +++++++++++++++++++++++++-----
 1 file changed, 55 insertions(+), 11 deletions(-)

diff --git a/Machines/Thomson/MO5/MemoryMap.hpp b/Machines/Thomson/MO5/MemoryMap.hpp
index af9da4c8f..36bdf4605 100644
--- a/Machines/Thomson/MO5/MemoryMap.hpp
+++ b/Machines/Thomson/MO5/MemoryMap.hpp
@@ -19,6 +19,20 @@
 
 namespace Thomson::MO5 {
 
+//
+// Notes on the layout of the standard MO ROM images:
+//
+//	The MO5 images are 16kb in total, representing what should go into the
+//	final 16kb of the memory map.
+//
+//	The MO6 images are 64kb in total:
+//
+//	* the first 12kb is the BASIC 1 portion;
+//	* the next 12 kb is <something>, possibly the GUI?
+//	* what is possibly a 4kb gap then follows — it's less than 300 bytes of content, then filler — though it ends in C000 which could be a vector?;
+//	* there are then probably two meaningful pages of a full 16kb each (the first of which is definitely to do with BASIC 128, presumably the second also); and
+//	* the final 4kb is the monitor, which resides permanently at $f000–;
+
 template <bool is_mo6>
 struct MemoryMap {
 public:
@@ -50,9 +64,12 @@ public:
 	}
 
 	void set_cartridge(const std::vector<uint8_t> &cartridge) {
+		cartridge_ = cartridge;
 		if constexpr (!is_mo6) {
-			set_rom(cartridge.begin(), cartridge.end());
+			cartridge_is_paged_ = true;
+			rom_.clear();
 		}
+		update_commutable_rom();
 	}
 
 	void set_floppy_rom(const std::vector<uint8_t> &rom) {
@@ -109,17 +126,33 @@ private:
 	const uint8_t *read_[0x10]{};
 
 	void update_commutable_rom() {
-		// TODO: this should be pageable.
-
-		// Commutable ROM: 0xb000 to 0xf000; if it's smaller than that then
-		// align it to end at 0xf000.
-		const size_t start = 0xf - (rom_.size() >> 12);
-		for(size_t c = 0xb; c < 0xf; c++) {
-			if(c >= start) {
-				set_read(c, rom_.data() + ((c - start) << 12));
-			} else {
-				set_read(c, nullptr);
+		const auto set_range = [&](const size_t start, const size_t length, const uint8_t *source) {
+			for(size_t c = 0x0; c < length; c++) {
+				set_read(c + start, source + (c << 12));
 			}
+		};
+
+		if(cartridge_is_paged_) {
+			// TODO: there's some sort of paging here, too.
+			set_range(0xb, 0x4, cartridge_.data());
+			return;
+		}
+
+		switch(paged_rom_) {
+			case PagedROM::FirstBank12kb:
+				set_read(0xb, nullptr);
+				set_range(0xc, 0x3, rom_.data());
+			break;
+			case PagedROM::SecondBank12kb:
+				set_read(0xb, nullptr);
+				set_range(0xc, 0x3, rom_.data() + 12*1024);
+			break;
+			case PagedROM::ThirdBank16kb:
+				set_range(0xb, 0x4, rom_.data() + 28*1024);
+			break;
+			case PagedROM::FourthBank16kb:
+				set_range(0xb, 0x4, rom_.data() + 44*1024);
+			break;
 		}
 	}
 
@@ -136,6 +169,17 @@ private:
 	std::array<uint8_t, 0x7c0> floppy_rom_;
 	std::array<uint8_t, 0x1000> monitor_;
 	std::vector<uint8_t> rom_;
+	std::vector<uint8_t> cartridge_;
+
+	// TODO: the following are entirely invented by me. Probably will need adjusting to
+	// whatever the hardware registers model, if I ever figure it out.
+	enum class PagedROM {
+		FirstBank12kb = 0,
+		SecondBank12kb = 1,
+		ThirdBank16kb = 2,
+		FourthBank16kb = 3,
+	} paged_rom_ = is_mo6 ? PagedROM::SecondBank12kb : PagedROM::FirstBank12kb;
+	bool cartridge_is_paged_ = false;
 };
 
 }