diff --git a/src/northbridge/intel/haswell/bootblock.c b/src/northbridge/intel/haswell/bootblock.c
index 59c31aa..64e2258 100644
--- a/src/northbridge/intel/haswell/bootblock.c
+++ b/src/northbridge/intel/haswell/bootblock.c
@@ -1,25 +1,34 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
 #include <arch/bootblock.h>
+#include <assert.h>
 #include <device/pci_ops.h>
+#include <types.h>
 #include "haswell.h"
 
+static uint32_t encode_pciexbar_length(void)
+{
+	switch (CONFIG_MMCONF_BUS_NUMBER) {
+		case 256: return 0 << 1;
+		case 128: return 1 << 1;
+		case  64: return 2 << 1;
+		default:  return dead_code_t(uint32_t);
+	}
+}
+
 void bootblock_early_northbridge_init(void)
 {
-	uint32_t reg;
-
 	/*
-	 * The "io" variant of the config access is explicitly used to setup the PCIEXBAR
-	 * because CONFIG(MMCONF_SUPPORT) is set to true. That way, all subsequent
-	 * non-explicit config accesses use MCFG. This code also assumes that
-	 * bootblock_northbridge_init() is the first thing called in the non-asm
-	 * boot block code. The final assumption is that no assembly code is using
-	 * the CONFIG(MMCONF_SUPPORT) option to do PCI config acceses.
+	 * The "io" variant of the config access is explicitly used to setup the
+	 * PCIEXBAR because CONFIG(MMCONF_SUPPORT) is set to true. That way, all
+	 * subsequent non-explicit config accesses use MCFG. This code also assumes
+	 * that bootblock_northbridge_init() is the first thing called in the non-asm
+	 * boot block code. The final assumption is that no assembly code is using the
+	 * CONFIG(MMCONF_SUPPORT) option to do PCI config accesses.
 	 *
 	 * The PCIEXBAR is assumed to live in the memory mapped IO space under 4GiB.
 	 */
-	reg = 0;
-	pci_io_write_config32(HOST_BRIDGE, PCIEXBAR + 4, reg);
-	reg = CONFIG_MMCONF_BASE_ADDRESS | 4 | 1; /* 64MiB - 0-63 buses. */
+	const uint32_t reg = CONFIG_MMCONF_BASE_ADDRESS | encode_pciexbar_length() | 1;
+	pci_io_write_config32(HOST_BRIDGE, PCIEXBAR + 4, 0);
 	pci_io_write_config32(HOST_BRIDGE, PCIEXBAR, reg);
 }