diff --git a/src/arch/x86/isa/decoder/one_byte_opcodes.isa b/src/arch/x86/isa/decoder/one_byte_opcodes.isa
index d6cfdc593..0225657c4 100644
--- a/src/arch/x86/isa/decoder/one_byte_opcodes.isa
+++ b/src/arch/x86/isa/decoder/one_byte_opcodes.isa
@@ -419,7 +419,7 @@
                 // extend it, and there's no easy way to specify only checking
                 // the first byte.
                 0x5: decode IMMEDIATE {
-                    0xffffffffffffff80:
+                    -0x80:
                          SyscallInst::int80('xc->syscall(Rax)', IsSyscall);
                     default: int_Ib();
                 }