[mubios] Have IRQ0 maintain the BIOS clock, and implement INT 1Ah AH=00h.

Signed-off-by: Richard Qin <richard.qin.x@gmail.com>

3 files changed, 82 insertions, 0 deletions

diff --git a/mubios/arch/interrupt.S b/mubios/arch/interrupt.S
new file mode 100644
index 0000000..7cb3f4b
--- /dev/null
+++ b/mubios/arch/interrupt.S
@@ -0,0 +1,50 @@
+# RCC-Project
+#
+# Copyright (C) 2026 Richard Qin
+#
+# This file is part of RCC-Project.
+#
+# RCC-Project is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation; either version 3 of the License, or (at your option) any
+# later version.
+#
+# RCC-Project is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# RCC-Project; see the file COPYING.  If not see
+# <http://www.gnu.org/licenses/>.
+
+	.intel_syntax noprefix
+	.code16
+
+	.section .text16.irq, "ax"
+
+	.global int1a_entry
+	.type int1a_entry, @function
+
+int1a_entry:
+	cmp ah, 0x00
+	jne int1a_return
+
+	push ds
+	push bx
+
+	xor bx, bx
+	mov ds, bx
+
+	mov bx, 0x046C
+	mov cx, word ptr [bx + 2]
+	mov dx, word ptr [bx]
+
+	mov al, byte ptr [0x0470]
+	mov byte ptr [0x0470], 0
+	mov ah, 0x00
+
+	pop bx
+	pop ds
+
+int1a_return:
+	iret
diff --git a/mubios/arch/irq.S b/mubios/arch/irq.S
index 6eea350..dbde8ad 100644
--- a/mubios/arch/irq.S
+++ b/mubios/arch/irq.S
@@ -20,6 +20,12 @@
 	.intel_syntax noprefix
 	.code16
 
+	.equ BDA_TIMER_COUNTER, 0x046C
+	.equ BDA_TIMER_ROLLOVER, 0x0470
+
+	.equ TIMER_TICKS_PER_DAY_LOW, 0x00B0
+	.equ TIMER_TICKS_PER_DAY_HIGH, 0x0018
+
 	.section .text16.irq, "ax"
 
 	.global irq0_entry
@@ -27,11 +33,34 @@
 
 irq0_entry:
 	push ax
+	push bx
+
+	xor bx, bx
+	mov ds, bx
+
+	mov bx, BDA_TIMER_COUNTER
+	inc word ptr [bx]
+
+	jnz irq0_send_eoi
+
+	inc word ptr [bx + 2]
+
+	cmp word ptr [bx + 2], TIMER_TICKS_PER_DAY_HIGH
+	jne irq0_send_eoi
+
+	cmp word ptr [bx], TIMER_TICKS_PER_DAY_LOW
+	jne irq0_send_eoi
+
+	mov word ptr [bx], 0
+	mov word ptr [bx + 2], 0
+	mov byte ptr [BDA_TIMER_ROLLOVER], 1
 
+irq0_send_eoi:
 	# Acknowledge IRQ0 on the master PIC.
 	mov al, 0x20
 	out 0x20, al
 
+	pop bx
 	pop ax
 	iret
 
diff --git a/mubios/core/bda.c b/mubios/core/bda.c
index c2ecfb1..547544b 100644
--- a/mubios/core/bda.c
+++ b/mubios/core/bda.c
@@ -56,6 +56,8 @@ extern u8 __rom_start[];
 extern void irq0_entry (void);
 extern void irq1_entry (void);
 
+extern void int1a_entry (void);
+
 static void
 install_ivt_entry (u8 vector, void (*handler) (void))
 {
@@ -76,4 +78,5 @@ ivt_init (void)
 {
   install_ivt_entry (0x08u, irq0_entry);
   install_ivt_entry (0x09u, irq1_entry);
+  install_ivt_entry (0x1Au, int1a_entry);
 }