diff --git a/modules/outputs/tftpbootlz.nix b/modules/outputs/tftpbootlz.nix
index 12fc997..d8fb449 100644
--- a/modules/outputs/tftpbootlz.nix
+++ b/modules/outputs/tftpbootlz.nix
@@ -10,34 +10,52 @@ let
   o = config.system.outputs;
   cmdline = concatStringsSep " " config.boot.commandLine;
   cfg = config.boot.tftp;
-  boot-scr =
-    pkgs.buildPackages.runCommand "boot-scr" { nativeBuildInputs = [ pkgs.pkgsBuildBuild.dtc ];  } ''
-      uimageSize=$(($(stat -L -c %s ${o.zimage}) + 0x1000 &(~0xfff)))
-      rootfsStart=0x$(printf %x $((${toString cfg.loadAddress} + 0x100000 + $uimageSize   &(~0xfffff) )))
-      rootfsBytes=$(($(stat -L -c %s ${o.rootfs}) + 0x100000 &(~0xfffff)))
-      rootfsBytes=$(($rootfsBytes + ${toString cfg.freeSpaceBytes} ))
-      rootfsMb=$(($rootfsBytes >> 20))
-      cmd="mtdparts=phram0:''${rootfsMb}M(rootfs) phram.phram=phram0,''${rootfsStart},''${rootfsBytes},${toString config.hardware.flash.eraseBlockSize} root=/dev/mtdblock0";
+  tftpbootlz =
+    pkgs.buildPackages.runCommand "tftpbootlz" {
+      nativeBuildInputs = with pkgs.pkgsBuildBuild; [ lzma dtc ];
+    } ''
+      binsize() { local s=$(stat -L -c %s $1); echo $(($s + 0x1000 &(~0xfff))); }
+      binsize64k() { local s=$(stat -L -c %s $1); echo $(($s + 0x10000 &(~0xffff))); }
+      hex() { printf "0x%x" $1; }
 
-      dtbStart=$(printf %x $((${toString cfg.loadAddress} + $rootfsBytes + 0x100000 + $uimageSize )))
-
-      mkdir $out
+      kernelStart=${toString cfg.loadAddress}
+      kernelSize=$(binsize ${o.zimage})
+      mkdir -p $out
       cat ${o.dtb} > $out/dtb
+      fdtput -p -t lx $out/dtb /reserved-memory '#address-cells' 1
+      fdtput -t lx $out/dtb /reserved-memory '#size-cells' 1
+      fdtput $out/dtb /reserved-memory ranges
       fdtput -p -t s $out/dtb /reserved-memory/phram-rootfs compatible phram
-      fdtput -p -t lx $out/dtb /reserved-memory/phram-rootfs reg 0 $rootfsStart 0 $(printf %x $rootfsBytes)
+      # can't calculate the actual address here until we know how
+      # big the dtb will be
+      fdtput -t lx $out/dtb /reserved-memory/phram-rootfs reg 0xdead 0xcafe
+      dtbStart=$(($kernelStart + $kernelSize))
+      dtbSize=$(binsize $out/dtb)
+      rootfsOrigSize=$(binsize64k ${o.rootfs})
+      lzma -z9cv ${o.rootfs} > $out/rootfs.lz
 
-      dtbBytes=$(($(stat -L -c %s $out/dtb) + 0x1000 &(~0xfff)))
+      rootfsLzSize=$(binsize $out/rootfs.lz)
+      rootfsLzStart=$(($dtbStart + $dtbSize))
+      rootfsOrigStart=$(($rootfsLzStart + $rootfsLzSize))
+      fdtput -t lx $out/dtb /reserved-memory/phram-rootfs reg $(printf "%x" $rootfsOrigStart) $(printf "%x"  $rootfsOrigSize)
+      #  dtc -I dtb  -O dts  -o /dev/stdout $out/dtb ; exit 1
 
-      cat > $out/script << EOF
+      cmd="mtdparts=phram0:''${rootfsOrigSize}(rootfs) phram.phram=phram0,''${rootfsOrigStart},''${rootfsOrigSize},${toString config.hardware.flash.eraseBlockSize} root=/dev/mtdblock0";
+
+      (cd $out;
+      ln -s ${o.zimage} zImage
+      ln -s ${o.manifest} manifest
+      ln -s ${o.kernel.headers} build)
+
+      cat > $out/boot.scr << EOF
       setenv serverip ${cfg.serverip}
       setenv ipaddr ${cfg.ipaddr}
-      setenv bootargs 'liminix ${cmdline} $cmd'
-      tftpboot 0x${lib.toHexString cfg.loadAddress} result/zImage; tftpboot 0x$(printf %x $rootfsStart) result/rootfs ; tftpboot 0x$dtbStart result/dtb
-      setexpr bootaddr 0x$dtbStart + \$filesize
-      # lzmadec 0x${lib.toHexString cfg.loadAddress}  \$bootaddr
-      # bootz \$bootaddr - 0x$dtbStart
-      bootz 0x${lib.toHexString cfg.loadAddress} - 0x$dtbStart
+      setenv bootargs "liminix ${cmdline} $cmd"
+      tftpboot $(hex $kernelStart) result/zImage; tftpboot $(hex $dtbStart) result/dtb ; tftpboot $(hex $rootfsLzStart) result/rootfs.lz
+      lzmadec $(hex $rootfsLzStart) $(hex $rootfsOrigStart)
+      bootz $(hex $kernelStart) - $(hex $dtbStart)
       EOF
+
     '';
 in {
   imports = [ ../ramdisk.nix ];
@@ -53,35 +71,20 @@ in {
         **********
 
         This is a variant of the tftpboot output intended for the
-        Turris Omnia. It builds a uimage containing an uncompressed
-        kernel, then compresses the resulting image to be decompressed
-        using the u-boot lzmadec command. This is a workaround for an
-        awkwardly low CONFIG_SYS_BOOTM_LEN setting in the U-Boot build
-        for the device, which means that the regular tftpboot output
-        would only work for very small kernels.
+        Turris Omnia. It uses a zImage instead of a uimage, as a
+        workaround for an awkwardly low CONFIG_SYS_BOOTM_LEN setting
+        in the U-Boot build for the device which means it won't boot
+        uimages unless they're teeny tiny.
+
+        As a bonus, it also uses lzma compression on the rootfs,
+        which reduces a 20MB ext4 image to around 4MB
       '';
     };
   };
   config = {
     boot.ramdisk.enable = true;
-
     system.outputs = {
-      tftpbootlz =
-        let
-          o = config.system.outputs; in
-          pkgs.runCommand "tftpboot" {
-            depsBuildBuild = [ pkgs.pkgsBuildBuild.lzma ];
-          } ''
-          mkdir $out
-          cd $out
-          ln -s ${o.rootfs} rootfs
-          ln -s ${o.zimage} zImage
-          ln -s ${o.manifest} manifest
-          ln -s ${o.kernel.headers} build
-          # lzma -c -z -9  < {uimage} > uimage.lz
-          ln -s ${boot-scr}/dtb dtb
-          ln -s ${boot-scr}/script boot.scr
-       '';
+      inherit tftpbootlz;
     };
   };
 }