diff --git a/src/arch/x86/prefix/usbdisk.S b/src/arch/x86/prefix/usbdisk.S
index 9676406e2..830f3774c 100644
--- a/src/arch/x86/prefix/usbdisk.S
+++ b/src/arch/x86/prefix/usbdisk.S
@@ -9,26 +9,53 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
 #include "mbr.S"
 
 /* Partition table: 64 heads, 32 sectors/track (ZIP-drive compatible) */
+#define HEADS 64
+#define SECTORS 32
+#define CYLADDR(cyl) ((((cyl) * HEADS + (((cyl) == 0) & 1)) * SECTORS) * 512)
+
+#define LOGSTART 0
+#define LOGCOUNT 1
+#define BOOTSTART 1
+#define BOOTCOUNT 2
+
+	/* Construct a C/H/S address */
+	.macro	chs cylinder, head, sector
+	.byte	\head
+	.byte	(((\cylinder & 0x300) >> 2) | \sector)
+	.byte	(\cylinder & 0x0ff)
+	.endm
+
+	/* Construct a linear address */
+	.macro	linear cylinders, heads, sectors
+	.long	((((\cylinders * HEADS) + \heads) * SECTORS) + \sectors - 1)
+	.endm
+
+	/* Construct a partition table entry */
+	.macro	partition bootflag, type, start, count
+	.byte	\bootflag
+	chs	\start, ((\start == 0) & 1), 1
+	.byte	\type
+	chs	(\start + \count - 1), (HEADS - 1), SECTORS
+	linear	\start, ((\start == 0) & 1), 1
+	linear	\count, 0, (1 - (((\start == 0) & 1) * SECTORS))
+	.endm
+
+	/* Partition table */
 	.org 446
 	.space 16
 	.space 16
 	/* Partition 3: log partition (for CONSOLE_INT13) */
-	.byte 0x00, 0x01, 0x01, 0x00
-	.byte 0xe0, 0x3f, 0x20, 0x00
-	.long 0x00000020
-	.long 0x000007e0
+	partition 0x00, 0xe0, LOGSTART, LOGCOUNT
 	/* Partition 4: boot partition */
-	.byte 0x80, 0x00, 0x01, 0x01
-	.byte 0xeb, 0x3f, 0x20, 0x02
-	.long 0x00000800
-	.long 0x00001000
+	partition 0x80, 0xeb, BOOTSTART, BOOTCOUNT
 
+	/* Disk signature */
 	.org 510
 	.byte 0x55, 0xaa
 
 /* Skip to start of log partition */
-	.org 32 * 512
+	.org CYLADDR(LOGSTART)
 	.ascii "iPXE LOG\n\n"
 
 /* Skip to start of boot partition */
-	.org 2048 * 512
+	.org CYLADDR(BOOTSTART)