From 30c2c65b779f13376febd5343bcfa88c42651daa Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Wed, 17 Apr 2024 21:36:39 -0400
Subject: [PATCH] Eliminate hot-path switch.

---
 .../Acorn/Archimedes/MemoryController.hpp     | 46 ++++++++++---------
 1 file changed, 24 insertions(+), 22 deletions(-)

diff --git a/Machines/Acorn/Archimedes/MemoryController.hpp b/Machines/Acorn/Archimedes/MemoryController.hpp
index a5e8828e4..6e91b8ef2 100644
--- a/Machines/Acorn/Archimedes/MemoryController.hpp
+++ b/Machines/Acorn/Archimedes/MemoryController.hpp
@@ -107,6 +107,25 @@ struct MemoryController {
 						high_rom_access_time_ = ROMAccessTime((address >> 6) & 3);
 						low_rom_access_time_ = ROMAccessTime((address >> 4) & 3);
 						page_size_ = PageSize((address >> 2) & 3);
+						switch(page_size_) {
+							default:
+							case PageSize::kb4:
+								page_address_shift_ = 12;
+								page_adddress_mask_ = 0x0fff;
+							break;
+							case PageSize::kb8:
+								page_address_shift_ = 13;
+								page_adddress_mask_ = 0x1fff;
+							break;
+							case PageSize::kb16:
+								page_address_shift_ = 14;
+								page_adddress_mask_ = 0x3fff;
+							break;
+							case PageSize::kb32:
+								page_address_shift_ = 15;
+								page_adddress_mask_ = 0x7fff;
+							break;
+						}
 
 						logger.info().append("MEMC Control: %08x -> OS:%d sound:%d video:%d refresh:%d high:%d low:%d size:%d", address, os_mode_, sound_dma_enable_, video_dma_enable_, dynamic_ram_refresh_, high_rom_access_time_, low_rom_access_time_, page_size_);
 						map_dirty_ = true;
@@ -296,6 +315,8 @@ struct MemoryController {
 			kb16 = 0b10,
 			kb32 = 0b11,
 		} page_size_ = PageSize::kb4;
+		int page_address_shift_ = 12;
+		uint32_t page_adddress_mask_ = 0xffff;
 
 		// Address translator.
 		//
@@ -337,33 +358,14 @@ struct MemoryController {
 			}
 			address = aligned<IntT>(address);
 			address &= 0x1ff'ffff;
-			size_t page;
-
-			// TODO: eliminate switch here.
-			switch(page_size_) {
-				default:
-				case PageSize::kb4:
-					page = address >> 12;
-					address &= 0x0fff;
-				break;
-				case PageSize::kb8:
-					page = address >> 13;
-					address &= 0x1fff;
-				break;
-				case PageSize::kb16:
-					page = address >> 14;
-					address &= 0x3fff;
-				break;
-				case PageSize::kb32:
-					page = address >> 15;
-					address &= 0x7fff;
-				break;
-			}
+			const size_t page = address >> page_address_shift_;
 
 			const auto &map = mapping<is_read>(trans, os_mode_);
 			if(!map[page]) {
 				return nullptr;
 			}
+
+			address &= page_adddress_mask_;
 			return reinterpret_cast<IntT *>(&map[page][address]);
 		}