From a99577d2eba3a0cb0396071fcc6b4a1132e4ef13 Mon Sep 17 00:00:00 2001
From: OBattler <oubattler@gmail.com>
Date: Fri, 19 Dec 2025 22:30:28 +0100
Subject: [PATCH] ABit AB-BX6: Force bit 4 of CMOS register 39h to 1, fixes
 intermittent POST freezes on POST code AAh on later BIOS revisions.

---
 src/nvr_at.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/src/nvr_at.c b/src/nvr_at.c
index ae7bd8bb4..2e04ddc6f 100644
--- a/src/nvr_at.c
+++ b/src/nvr_at.c
@@ -635,6 +635,13 @@ nvr_reg_write(uint16_t reg, uint8_t val, void *priv)
             nvr_reg_common_write(reg, val, nvr, local);
             break;
 
+        case 0x39:
+            if (machines[machine].init == machine_at_bx6_init)
+                nvr_reg_common_write(reg, val | 0x08, nvr, local);
+            else
+                nvr_reg_common_write(reg, val, nvr, local);
+            break;
+
         default: /* non-RTC registers are just NVRAM */
             nvr_reg_common_write(reg, val, nvr, local);
             break;
@@ -794,6 +801,14 @@ nvr_read(uint16_t addr, void *priv)
                     ret = nvr->regs[local->addr[addr_id]];
                 break;
 
+            case 0x39:
+                if (!(local->lock[local->addr[addr_id]] & 0x02)) {
+                    ret = nvr->regs[local->addr[addr_id]];
+                    if (machines[machine].init == machine_at_bx6_init)
+                        ret |= 0x08;
+                }
+                break;
+
             case 0x3e:
             case 0x3f:
                 if (!nvr->is_new && (local->flags & FLAG_AMI_1995_HACK)) {