commit 4637394ed6bb6399e2f1c640db14df56752888c3
Author: Matt Jenkins <matt@majenko.co.uk>
Date:   Tue Mar 24 15:21:33 2026 +0000

    Initial import

diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..a70bf21
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,2 @@
+vaxcheck: vaxcheck.o
+	$(CC) -o $@ $^
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..632b665
--- /dev/null
+++ b/README.md
@@ -0,0 +1,11 @@
+VaxCheck
+========
+
+Calculate (and optionally write) VAX ROM checksums
+
+The method used is a simple rotate-left-and-add over the
+bytes of a ROM image, excluding the final checksum byte.
+
+This is done on a per physical ROM basis, not for the
+whole ROM memory space as a whole, so there are four
+checksum bytes, one per physical ROM.
diff --git a/vaxcheck.c b/vaxcheck.c
new file mode 100644
index 0000000..12aecf9
--- /dev/null
+++ b/vaxcheck.c
@@ -0,0 +1,77 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+
+int main(int argc, char **argv) {
+	if (argc < 2) {
+		printf("Usage: vaxcheck <filename.rom> [<output.rom>]\n");
+		return -1;
+	}
+
+	if (argc > 3) {
+		printf("Usage: vaxcheck <filename.rom> [<output.rom>]\n");
+		return -1;
+	}
+
+	int f = open(argv[1], O_RDONLY);
+
+	if (!f) {
+		printf("Unable to open %s for reading\n", argv[1]);
+		return -1;
+	}
+
+	lseek(f, 3, SEEK_SET);
+
+	uint8_t v[5];
+	read(f, v, 5);
+	if ((v[1] != 0x55) ||
+		(v[2] != 0xAA) ||
+		(v[3] != 0x33)) {
+		printf("File is not a VAX ROM image\n");
+		return -1;
+	}
+
+	uint32_t romsize = v[4] << 10;
+
+	printf("ROM is a %d byte VAX ROM image, with rom ID of %d\n", romsize, v[0]);
+
+	lseek(f, 0, SEEK_SET);
+
+	int out = -1;
+	if (argc == 3) {
+		out = open(argv[2], O_RDWR | O_TRUNC | O_CREAT, 0644);
+		if (!out) {
+			printf("Unable to open %s for writing\n", argv[2]);
+		}
+	}
+
+	uint32_t r2 = 0;
+	uint8_t r3 = 0; // Accumulator
+	uint8_t r4 = 0; // Test byte
+	for (r2 = 0; r2 < (romsize-1); r2++) {
+		read(f, &r4, 1);
+		if (out) {
+			write(out, &r4, 1);
+		}
+		uint8_t tmp = r3 >> 7;
+		r3 = (r3 << 1) | tmp;
+		r3 = r3 + r4;
+	}
+
+	uint8_t cs;
+	read(f, &cs, 1);
+
+	printf("Current checksum:    0x%02x\n", cs);
+	printf("Calculated checksum: 0x%02x\n", r3);
+
+	if (out) {
+		write(out, &r3, 1);
+		close(out);
+	}
+	close(f);
+
+	return !(r3 == cs);
+}