From d3618e65752e8c675848e1642e0709cc3bb7a40e Mon Sep 17 00:00:00 2001
From: OBattler <oubattler@gmail.com>
Date: Sat, 2 May 2020 05:41:38 +0200
Subject: [PATCH] Fixes to mem.c internal operation, non-RAM areas of the
 memory space should now be handled more sanely.

---
 src/mem.c | 50 ++++++++++++++++++++++++++++----------------------
 1 file changed, 28 insertions(+), 22 deletions(-)

diff --git a/src/mem.c b/src/mem.c
index a145c9631..ddd6a2316 100644
--- a/src/mem.c
+++ b/src/mem.c
@@ -680,7 +680,7 @@ writemembl(uint32_t addr, uint8_t val)
     mem_mapping_t *map;
     mem_logical_addr = addr;
 
-    if (page_lookup[addr>>12]) {
+    if (page_lookup[addr>>12] && page_lookup[addr>>12]->write_b) {
 	page_lookup[addr>>12]->write_b(addr, val, page_lookup[addr>>12]);
 	return;
     }
@@ -772,7 +772,7 @@ writememwl(uint32_t addr, uint16_t val)
 	}
     }
 
-    if (page_lookup[addr>>12]) {
+    if (page_lookup[addr>>12] && page_lookup[addr>>12]->write_w) {
 	page_lookup[addr>>12]->write_w(addr, val, page_lookup[addr>>12]);
 	return;
     }
@@ -874,7 +874,7 @@ writememll(uint32_t addr, uint32_t val)
 		return;
 	}
     }
-    if (page_lookup[addr>>12]) {
+    if (page_lookup[addr>>12] && page_lookup[addr>>12]->write_l) {
 	page_lookup[addr>>12]->write_l(addr, val, page_lookup[addr>>12]);
 	return;
     }
@@ -970,7 +970,7 @@ writememql(uint32_t addr, uint64_t val)
 		return;
 	}
     }
-    if (page_lookup[addr>>12]) {
+    if (page_lookup[addr>>12] && page_lookup[addr>>12]->write_l) {
 	page_lookup[addr>>12]->write_l(addr, val, page_lookup[addr>>12]);
 	page_lookup[addr>>12]->write_l(addr + 4, val >> 32, page_lookup[addr>>12]);
 	return;
@@ -1104,7 +1104,7 @@ writememwl(uint32_t seg, uint32_t addr, uint16_t val)
 	}
     }
 
-    if (page_lookup[addr2>>12]) {
+    if (page_lookup[addr2>>12] && page_lookup[addr2>>12]->write_w) {
 	page_lookup[addr2>>12]->write_w(addr2, val, page_lookup[addr2>>12]);
 	return;
     }
@@ -1209,7 +1209,7 @@ writememll(uint32_t seg, uint32_t addr, uint32_t val)
 	}
     }
 
-    if (page_lookup[addr2>>12]) {
+    if (page_lookup[addr2>>12] && page_lookup[addr2>>12]->write_l) {
 	page_lookup[addr2>>12]->write_l(addr2, val, page_lookup[addr2>>12]);
 	return;
     }
@@ -1307,7 +1307,7 @@ writememql(uint32_t seg, uint32_t addr, uint64_t val)
 	}
     }
 
-    if (page_lookup[addr2>>12]) {
+    if (page_lookup[addr2>>12] && page_lookup[addr2>>12]->write_l) {
 	page_lookup[addr2>>12]->write_l(addr2, val, page_lookup[addr2>>12]);
 	page_lookup[addr2>>12]->write_l(addr2 + 4, val >> 32, page_lookup[addr2>>12]);
 	return;
@@ -2303,7 +2303,7 @@ mem_a20_init(void)
 void
 mem_reset(void)
 {
-    uint32_t c, m;
+    uint32_t c, m, m2;
 
     m = 1024UL * mem_size;
     if (ram != NULL) {
@@ -2324,23 +2324,21 @@ mem_reset(void)
 		m = 4096;
 	} else {
 		/* 80386+; maximum address space is 4GB. */
-		if (is486) {
-			/* We need this since there might be BIOS execution at the end of RAM,
-			   which could break the recompiler if there's not enough page elements. */
-			m = 1048576;
-		} else {
-			m = (mem_size + 384) >> 2;
-			if ((m << 2) < (mem_size + 384))
-				m++;
-			if (m < 4096)
-				m = 4096;
-		}
+		m = 1048576;
 	}
     } else {
 	/* 8088/86; maximum address space is 1MB. */
 	m = 256;
     }
 
+    /* Calculate the amount of pages used by RAM, so that we can
+       give all the pages above this amount NULL write handlers. */
+    m2 = (mem_size + 384) >> 2;
+    if ((m2 << 2) < (mem_size + 384))
+	m2++;
+    if (m2 < 4096)
+	m2 = 4096;
+
     /*
      * Allocate and initialize the (new) page table.
      * We only do this if the size of the page table has changed.
@@ -2399,9 +2397,17 @@ mem_log("MEM: reset: new pages=%08lx, pages_sz=%i\n", pages, pages_sz);
 
     for (c = 0; c < pages_sz; c++) {
 	pages[c].mem = &ram[c << 12];
-	pages[c].write_b = mem_write_ramb_page;
-	pages[c].write_w = mem_write_ramw_page;
-	pages[c].write_l = mem_write_raml_page;
+	if (c < m) {
+		pages[c].write_b = mem_write_ramb_page;
+		pages[c].write_w = mem_write_ramw_page;
+		pages[c].write_l = mem_write_raml_page;
+	} else {
+		/* Make absolute sure non-RAM pages have NULL handlers so the
+		   memory read/write handlers know to ignore them. */
+		pages[c].write_b = NULL;
+		pages[c].write_w = NULL;
+		pages[c].write_l = NULL;
+	}
 #ifdef USE_NEW_DYNAREC
 	pages[c].evict_prev = EVICT_NOT_IN_LIST;
 	pages[c].byte_dirty_mask = &byte_dirty_mask[c * 64];