diff --git a/src/agon_machine.rs b/src/agon_machine.rs
index 7140e82..d52fdcc 100644
--- a/src/agon_machine.rs
+++ b/src/agon_machine.rs
@@ -999,7 +999,9 @@ impl AgonMachine {
 
                 // save the size in the FIL structure
                 let mut file_len = f.seek(SeekFrom::End(0)).unwrap();
-                f.seek(SeekFrom::Start(0)).unwrap();
+                if mode & mos::FA_SEEKEND == 0 {
+                    f.seek(SeekFrom::Start(0)).unwrap();
+                }
 
                 // XXX don't support files larger than 512KiB
                 file_len = file_len.min(1<<19);
@@ -1018,7 +1020,6 @@ impl AgonMachine {
                     _ => cpu.state.reg.set24(Reg16::HL, 1)
                 }
             }
-
         }
         Environment::new(&mut cpu.state, self).subroutine_return();
     }
diff --git a/src/mos.rs b/src/mos.rs
index cd9f2eb..f1667b1 100644
--- a/src/mos.rs
+++ b/src/mos.rs
@@ -21,6 +21,7 @@ pub const FILINFO_MEMBER_FNAME_256BYTES: u32 = 22;
 pub const FA_WRITE: u32 = 2;
 pub const FA_CREATE_NEW: u32 = 4;
 pub const FA_CREATE_ALWAYS: u32 = 8;
+pub const FA_SEEKEND: u32 = 0x20;
 
 #[derive(Clone, Default)]
 pub struct MosMap {