diff --git a/sys/x86/iommu/amd_drv.c b/sys/x86/iommu/amd_drv.c
--- a/sys/x86/iommu/amd_drv.c
+++ b/sys/x86/iommu/amd_drv.c
@@ -217,6 +217,17 @@
 	sc->devtbl_obj = NULL;
 }
 
+static const int amdiommu_devtab_base_seg_regs[] = {
+	[0] = AMDIOMMU_DEVTAB_BASE,
+	[1] = AMDIOMMU_DEVTAB_S1_BASE,
+	[2] = AMDIOMMU_DEVTAB_S2_BASE,
+	[3] = AMDIOMMU_DEVTAB_S3_BASE,
+	[4] = AMDIOMMU_DEVTAB_S4_BASE,
+	[5] = AMDIOMMU_DEVTAB_S5_BASE,
+	[6] = AMDIOMMU_DEVTAB_S6_BASE,
+	[7] = AMDIOMMU_DEVTAB_S7_BASE,
+};
+
 static int
 amdiommu_create_dev_tbl(struct amdiommu_unit *sc)
 {
@@ -227,6 +238,7 @@
 	segnum_log = (sc->efr & AMDIOMMU_EFR_DEVTBLSEG_MASK) >>
 	    AMDIOMMU_EFR_DEVTBLSEG_SHIFT;
 	segnum = 1 << segnum_log;
+	MPASS(segnum <= nitems(amdiommu_devtab_base_seg_regs));
 
 	devtbl_sz = amdiommu_devtbl_sz(sc);
 	seg_sz = devtbl_sz / segnum;
@@ -276,9 +288,7 @@
 			pmap_zero_page(m);
 			pmap_qenter(seg_vaddr, &m, 1);
 		}
-		reg = i == 0 ? AMDIOMMU_DEVTAB_BASE : AMDIOMMU_DEVTAB_S1_BASE +
-		    ((i - 1) << 3);
-		amdiommu_write8(sc, reg, rval);		
+		amdiommu_write8(sc, amdiommu_devtab_base_seg_regs[i], rval);
 	}
 
 	return (0);