diff --git a/src/chipset/opti499.c b/src/chipset/opti499.c
index ed7c269b0..132754ac7 100644
--- a/src/chipset/opti499.c
+++ b/src/chipset/opti499.c
@@ -28,6 +28,7 @@
 #include <86box/device.h>
 #include <86box/mem.h>
 #include <86box/port_92.h>
+#include <86box/plat_fallthrough.h>
 #include <86box/plat_unused.h>
 #include <86box/chipset.h>
 
@@ -176,6 +177,9 @@ opti499_write(uint16_t addr, uint8_t val, void *priv)
                         break;
 
                     case 0x22:
+                        mem_a20_chipset = (val & 0x02);
+                        mem_a20_recalc();
+                        fallthrough;
                     case 0x23:
                     case 0x26:
                     case 0x2d:
diff --git a/src/include/86box/machine.h b/src/include/86box/machine.h
index ea5d56257..824183673 100644
--- a/src/include/86box/machine.h
+++ b/src/include/86box/machine.h
@@ -749,6 +749,9 @@ extern int             machine_at_4gpv5_init(const machine_t *);
 /* Contaq 82C597 */
 extern int             machine_at_greenb_init(const machine_t *);
 
+/* OPTi 499 */
+extern int             machine_at_xenon_init(const machine_t *);
+
 /* OPTi 895 */
 #ifdef EMU_DEVICE_H
 extern const device_t  j403tg_device;
diff --git a/src/include/86box/mem.h b/src/include/86box/mem.h
index 9051189a6..1fc95c047 100644
--- a/src/include/86box/mem.h
+++ b/src/include/86box/mem.h
@@ -307,6 +307,7 @@ extern int      read_type;
 
 extern int mem_a20_state;
 extern int mem_a20_alt;
+extern int mem_a20_chipset;
 extern int mem_a20_key;
 
 extern uint8_t  read_mem_b(uint32_t addr);
diff --git a/src/machine/m_at_socket3.c b/src/machine/m_at_socket3.c
index 008394505..bd044d65c 100644
--- a/src/machine/m_at_socket3.c
+++ b/src/machine/m_at_socket3.c
@@ -166,6 +166,29 @@ machine_at_greenb_init(const machine_t *model)
     return ret;
 }
 
+/* OPTi 499 */
+int
+machine_at_xenon_init(const machine_t *model)
+{
+    int ret;
+
+    ret = bios_load_linear("roms/machines/xenon/addx-bios-7-71-i28f001.bin",
+                           0x000e0000, 131072, 0);
+
+    if (bios_only || !ret)
+        return ret;
+
+    machine_at_common_init(model);
+
+    device_add(&opti499_device);
+    device_add(&ide_vlb_device);
+    device_add_params(&fdc37c6xx_device, (void *) (FDC37C661 | FDC37C6XX_IDE_PRI));
+    device_add_params(machine_get_kbc_device(machine), (void *) model->kbc_params);
+    device_add(&intel_flash_bxt_device);
+
+    return ret;
+}
+
 /* OPTi 895 */
 static const device_config_t j403tg_config[] = {
     // clang-format off
diff --git a/src/machine/machine_table.c b/src/machine/machine_table.c
index bf13025ff..c64e15af8 100644
--- a/src/machine/machine_table.c
+++ b/src/machine/machine_table.c
@@ -8761,6 +8761,50 @@ const machine_t machines[] = {
         .snd_device               = NULL,
         .net_device               = NULL
     },
+    /* Has AMIKey F KBC firmware. */
+    {
+        .name              = "[OPTi 499] ADD-X Normerel Xenon",
+        .internal_name     = "xenon",
+        .type              = MACHINE_TYPE_486_S3,
+        .chipset           = MACHINE_CHIPSET_OPTI_499,
+        .init              = machine_at_xenon_init,
+        .p1_handler        = machine_generic_p1_handler,
+        .gpio_handler      = NULL,
+        .available_flag    = MACHINE_AVAILABLE,
+        .gpio_acpi_handler = NULL,
+        .cpu               = {
+            .package     = CPU_PKG_SOCKET3,
+            .block       = CPU_BLOCK_NONE,
+            .min_bus     = 0,
+            .max_bus     = 0,
+            .min_voltage = 0,
+            .max_voltage = 0,
+            .min_multi   = 0,
+            .max_multi   = 0
+        },
+        .bus_flags = MACHINE_PS2_VLB,
+        .flags     = MACHINE_IDE | MACHINE_APM,
+        .ram       = {
+            .min  = 1024,
+            .max  = 65536,
+            .step = 1024
+        },
+        .nvrmask                  = 127,
+        .jumpered_ecp_dma         = 0,
+        .default_jumpered_ecp_dma = -1,
+        .kbc_device               = &kbc_at_device,
+        .kbc_params               = KBC_VEN_AMI | 0x00004600,
+        .kbc_p1                   = 0x00000cf0,
+        .gpio                     = 0xffffffff,
+        .gpio_acpi                = 0xffffffff,
+        .device                   = NULL,
+        .kbd_device               = NULL,
+        .fdc_device               = NULL,
+        .sio_device               = NULL,
+        .vid_device               = NULL,
+        .snd_device               = NULL,
+        .net_device               = NULL
+    },
     /* Version 1.0 has an AMIKEY-2, version 2.0 has a VIA VT82C42N KBC. */
     {
         .name              = "[OPTi 895] Jetway J-403TG",
@@ -18511,11 +18555,10 @@ const machine_t machines[] = {
             .block       = CPU_BLOCK(CPU_CYRIX3S),
             .min_bus     = 66666667,
             .max_bus     = 83333333,
-            /* TODO: to find the actual voltage and multiplier bus speeds. */
-            .min_voltage = 1800,
-            .max_voltage = 3500,
-            .min_multi   = 1.5,
-            .max_multi   = 8.0
+            .min_voltage = 2050,
+            .max_voltage = 3100,
+            .min_multi   = 3.5,
+            .max_multi   = 5.0
         },
         .bus_flags = MACHINE_PS2_PCI | MACHINE_BUS_USB,
         .flags     = MACHINE_IDE_DUAL | MACHINE_SOUND | MACHINE_APM | MACHINE_ACPI | MACHINE_USB,
diff --git a/src/mem/mem.c b/src/mem/mem.c
index 91fa277be..c456c840a 100644
--- a/src/mem/mem.c
+++ b/src/mem/mem.c
@@ -106,9 +106,10 @@ int cachesize = 256;
 uint32_t get_phys_virt;
 uint32_t get_phys_phys;
 
-int mem_a20_key   = 0;
-int mem_a20_alt   = 0;
-int mem_a20_state = 0;
+int mem_a20_key     = 0;
+int mem_a20_alt     = 0;
+int mem_a20_chipset = 0;
+int mem_a20_state   = 0;
 
 int mmuflush = 0;
 
@@ -3109,12 +3110,12 @@ mem_a20_recalc(void)
     if (!is286) {
         rammask = 0xfffff;
         flushmmucache();
-        mem_a20_key = mem_a20_alt = mem_a20_state = 0;
+        mem_a20_key = mem_a20_alt = mem_a20_state = mem_a20_chipset = 0;
 
         return;
     }
 
-    state = mem_a20_key | mem_a20_alt;
+    state = mem_a20_key | mem_a20_alt | mem_a20_chipset;
     if (state && !mem_a20_state) {
         rammask = cpu_16bitbus ? 0xffffff : 0xffffffff;
         if (is6117)