From f4c6221eb119254e47f051c9bcb1f675db4dc010 Mon Sep 17 00:00:00 2001
From: Rangi <35663410+Rangi42@users.noreply.github.com>
Date: Tue, 15 Sep 2026 20:22:33 -0400
Subject: [PATCH] Fix underflow of `ALIGN[16, offset]` (#2090)

---
 src/asm/section.cpp            | 13 +++++++++++--
 test/asm/align-beyond-size.asm |  3 +++
 test/asm/align-beyond-size.err |  3 +++
 3 files changed, 17 insertions(+), 2 deletions(-)
 create mode 100644 test/asm/align-beyond-size.asm
 create mode 100644 test/asm/align-beyond-size.err

diff --git a/src/asm/section.cpp b/src/asm/section.cpp
index 4acf8b52..5157a6ec 100644
--- a/src/asm/section.cpp
+++ b/src/asm/section.cpp
@@ -717,8 +717,17 @@ void sect_AlignPC(uint8_t alignment, uint16_t offset) {
 	} else if (alignment == 16) {
 		// Treat an alignment large enough as fixing the address.
 		// Note that this also ensures that a section's alignment never becomes 16 or greater.
-		sect->align = 0; // Reset the alignment, since we're fixing the address.
-		sect->org = offset - curOffset;
+		if (offset < curOffset) {
+			error(
+			    "Section already contains %" PRIu32
+			    " bytes, higher than this aligned address $%04" PRIx32,
+			    curOffset,
+			    offset
+			);
+		} else {
+			sect->align = 0; // Reset the alignment, since we're fixing the address.
+			sect->org = offset - curOffset;
+		}
 	} else if (alignment > sect->align) {
 		sect->align = alignment;
 		// We need `(sect->alignOfs + curOffset) & alignMask == offset`
diff --git a/test/asm/align-beyond-size.asm b/test/asm/align-beyond-size.asm
new file mode 100644
index 00000000..a570f6d9
--- /dev/null
+++ b/test/asm/align-beyond-size.asm
@@ -0,0 +1,3 @@
+SECTION "test", ROM0
+db 1, 2, 3
+align 16, 2
diff --git a/test/asm/align-beyond-size.err b/test/asm/align-beyond-size.err
new file mode 100644
index 00000000..73f24fc4
--- /dev/null
+++ b/test/asm/align-beyond-size.err
@@ -0,0 +1,3 @@
+error: Section already contains 3 bytes, higher than this aligned address $0002
+    at align-beyond-size.asm(3)
+Assembly aborted with 1 error