From 104d47e1a6f62c9f6418ba08ae74828766990c30 Mon Sep 17 00:00:00 2001
From: snow flurry <snow@datagirl.xyz>
Date: Mon, 15 Dec 2025 20:33:00 -0800
Subject: [PATCH] Initial commit

---
 README.md  | 11 +++++++
 dosbox.asm | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 102 insertions(+)
 create mode 100644 README.md
 create mode 100644 dosbox.asm

diff --git a/README.md b/README.md
new file mode 100644
index 0000000..59aac44
--- /dev/null
+++ b/README.md
@@ -0,0 +1,11 @@
+# DOS Tests
+
+Small COM programs to check quirks or functions of various DOSen to test which
+type they might be.
+
+Information on compiling them is available in the contents of each file.
+
+## dosbox.asm
+
+Tests for a special opcode only implemented on DOSBox (to my knowledge!). More
+info here: [https://datagirl.xyz/posts/dos_inside_the_box.html]
diff --git a/dosbox.asm b/dosbox.asm
new file mode 100644
index 0000000..b101340
--- /dev/null
+++ b/dosbox.asm
@@ -0,0 +1,91 @@
+org 100h
+bits 16
+
+section .text
+jmp _start
+
+yesDosbox   db "Yep, that is a DOSBox!",0dh,0ah,'$'
+noDosbox    db "Probably not a DOSBox.",0dh,0ah,'$'
+
+oldUDAddr   dw 0
+oldUDSeg    dw 0
+
+isDosbox    db 1
+
+_catchUD:
+    push ax
+    push bx
+
+    mov bx, sp
+    mov bx, WORD [ss:bx+4]
+    mov ax, bx
+
+    mov bx, WORD [cs:bx]
+    and bh, 38h ; Keep only the opcode bits
+    cmp bx, 38feh ; little-endian FE 38
+    je .notDosbox
+    ; uhh, we shouldn't end up here. clean up the IVT and IRET so the actual
+    ; #UD handler is called.
+    push es
+    xor ax, ax
+    mov es, ax
+    mov bx, [oldUDAddr]
+    mov [es:18h], bx
+    mov bx, [oldUDSeg]
+    mov [es:1ah], bx
+    pop es
+    jmp .catchDone
+
+    .notDosbox:
+    ; Not DOSBox -- increment the IP and unset the flag
+    add ax, 4
+    mov bx, sp
+    mov WORD [ss:bx+4], ax
+    mov [isDosbox], 0
+
+    .catchDone:
+        pop bx
+        pop ax
+        iret
+
+_start:
+    push es
+    ; store the old #UD...
+    xor ax, ax
+    mov es, ax
+    mov bx, [es:18h]
+    mov [oldUDAddr], bx
+    mov bx, [es:1ah]
+    mov [oldUDSeg], bx
+    ; ... and set the new #UD handler
+    mov bx, cs
+    mov [es:1ah], bx
+    mov bx, _catchUD
+    mov [es:18h], bx
+    pop es
+
+    ; DOSBoxCB 0x0000
+    db 0xfe, 0x38, 0x00, 0x00
+    mov ax, [isDosbox]
+    cmp ax, 0
+    jz .notDosbox
+    mov dx, yesDosbox
+    jmp .printAndQuit
+    .notDosbox:
+    mov dx, noDosbox
+    .printAndQuit:
+    mov ah, 09h
+    int 21h
+
+    ; restore the old #UD handler
+    push es
+    xor ax, ax
+    mov es, ax
+    mov [es:1ah], oldUDSeg
+    mov [es:18h], oldUDAddr
+    pop ax
+
+    ; and we're done!
+    mov ax, 4c00h
+    int 21h
+    hlt