From 9e7fc6a60d87fc38c75180873000a25b52d70601 Mon Sep 17 00:00:00 2001
From: OBattler <oubattler@gmail.com>
Date: Fri, 2 Feb 2024 17:52:00 +0100
Subject: [PATCH] DMA ports 81h and 82h workaround for proper CPU speed
 detection on the MegaPC BIOS and limited the MegaPC CPU speeds to >= 16 MHz
 and <= 25 MHz.

---
 src/dma.c                   | 46 ++++++++++++++++++++++++++++++-------
 src/machine/machine_table.c |  4 ++--
 2 files changed, 40 insertions(+), 10 deletions(-)

diff --git a/src/dma.c b/src/dma.c
index 55cf31236..1dcc3b25b 100644
--- a/src/dma.c
+++ b/src/dma.c
@@ -949,16 +949,46 @@ dma_page_read(uint16_t addr, UNUSED(void *priv))
     uint8_t convert[8] = CHANNELS;
     uint8_t ret        = 0xff;
 
-    addr &= 0x0f;
-    ret = dmaregs[2][addr];
+    if (((addr & 0xfffc) == 0x80) && (CS == 0xf000) &&
+        ((cpu_state.pc & 0xfffffff8) == 0x00007278) &&
+        !strcmp(machine_get_internal_name(), "megapc"))  switch (addr) {
+        /* The Amstrad MegaPC Quadtel BIOS times a sequence of:
+               mov ax,di
+               div bx
+           And expects this value to be at least 0x06e0 for 20 MHz,
+           and at least 0x0898 for 25 MHz, everything below 0x06e0
+           is assumed to be 16 MHz. Given that for some reason, this
+           does not occur on 86Box, we have to work around it here,
+           we return 0x0528 for 16 MHz, because it's the next in the
+           mathematical sequence (it equals 0x06e0 - (0x0898 - 0x06e0)). */
+        case 0x0081:
+            if (cpu_busspeed >= 25000000)
+                ret = 0x98;
+            else if (cpu_busspeed >= 20000000)
+                ret = 0xe0;
+            else
+                ret = 0x28;
+            break;
+        case 0x0082:
+            if (cpu_busspeed >= 25000000)
+                ret = 0x08;
+            else if (cpu_busspeed >= 20000000)
+                ret = 0x06;
+            else
+                ret = 0x05;
+            break;
+    } else {
+        addr &= 0x0f;
+        ret = dmaregs[2][addr];
 
-    if (addr >= 8)
-        addr = convert[addr & 0x07] | 4;
-    else
-        addr = convert[addr & 0x07];
+        if (addr >= 8)
+            addr = convert[addr & 0x07] | 4;
+        else
+            addr = convert[addr & 0x07];
 
-    if (addr < 8)
-        ret = dma[addr].page_l;
+        if (addr < 8)
+            ret = dma[addr].page_l;
+    }
 
     return ret;
 }
diff --git a/src/machine/machine_table.c b/src/machine/machine_table.c
index 0a0af878e..b45a7471c 100644
--- a/src/machine/machine_table.c
+++ b/src/machine/machine_table.c
@@ -4628,8 +4628,8 @@ const machine_t machines[] = {
         .cpu = {
             .package = CPU_PKG_386SX,
             .block = CPU_BLOCK_NONE,
-            .min_bus = 0,
-            .max_bus = 0,
+            .min_bus = 16000000,
+            .max_bus = 25000000,
             .min_voltage = 0,
             .max_voltage = 0,
             .min_multi = 0,