diff --git a/src/cpu/808x.c b/src/cpu/808x.c
index 0b6bd66b5..90563d9ab 100644
--- a/src/cpu/808x.c
+++ b/src/cpu/808x.c
@@ -1223,34 +1223,48 @@ static void
 add(int bits)
 {
     int size_mask = (1 << bits) - 1;
+    int special_case = 0;
+    uint32_t temp_src = cpu_src;
+
+    if ((cpu_alu_op == 2) && !(cpu_src & size_mask) && (cpu_state.flags & C_FLAG))
+        special_case = 1;
 
     cpu_data = cpu_dest + cpu_src;
+    if ((cpu_alu_op == 2) && (cpu_state.flags & C_FLAG))
+        cpu_src--;
     set_apzs(bits);
     set_of_add(bits);
 
     /* Anything - FF with carry on is basically anything + 0x100: value stays
        unchanged but carry goes on. */
-    if ((cpu_alu_op == 2) && !(cpu_src & size_mask) && (cpu_state.flags & C_FLAG))
+    if (special_case)
         cpu_state.flags |= C_FLAG;
     else
-        set_cf((cpu_src & size_mask) > (cpu_data & size_mask));
+        set_cf((temp_src & size_mask) > (cpu_data & size_mask));
 }
 
 static void
 sub(int bits)
 {
     int size_mask = (1 << bits) - 1;
+    int special_case = 0;
+    uint32_t temp_src = cpu_src;
+
+    if ((cpu_alu_op == 3) && !(cpu_src & size_mask) && (cpu_state.flags & C_FLAG))
+        special_case = 1;
 
     cpu_data = cpu_dest - cpu_src;
+    if ((cpu_alu_op == 3) && (cpu_state.flags & C_FLAG))
+        cpu_src--;
     set_apzs(bits);
     set_of_sub(bits);
 
     /* Anything - FF with carry on is basically anything - 0x100: value stays
        unchanged but carry goes on. */
-    if ((cpu_alu_op == 3) && !(cpu_src & size_mask) && (cpu_state.flags & C_FLAG))
+    if (special_case)
         cpu_state.flags |= C_FLAG;
     else
-        set_cf((cpu_src & size_mask) > (cpu_dest & size_mask));
+        set_cf((temp_src & size_mask) > (cpu_dest & size_mask));
 }
 
 static void
diff --git a/src/dma.c b/src/dma.c
index 55cf31236..50ae598b8 100644
--- a/src/dma.c
+++ b/src/dma.c
@@ -949,16 +949,47 @@ 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 0x0580 for 16 MHz, because it logically follows
+           in the sequence (0x06e0 = 0x0898 * (20 / 25), and
+           0x0580 = 0x06e0 * (16 / 20)). */
+        case 0x0081:
+            if (cpu_busspeed >= 25000000)
+                ret = 0x98;
+            else if (cpu_busspeed >= 20000000)
+                ret = 0xe0;
+            else
+                ret = 0x80;
+            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 c729a52b0..27eb05a09 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,