diff --git a/src/disasm.rs b/src/disasm.rs
index 9a53fe6..4c2a37b 100644
--- a/src/disasm.rs
+++ b/src/disasm.rs
@@ -468,7 +468,12 @@ impl Disassembler {
                     let imm = self.parse_byte();
                     Self::modrm_reg_to_grp1(reg, target, Operand::Byte(imm))
                 }
-                0x83 => panic!("Sign extented GPR1 not yet implemented"),
+                0x83 => {
+                    // byte extended version
+                    let (target, reg) = self.parse_modrm_byte(Operand::Word(0));
+                    let imm = self.parse_byte();
+                    Self::modrm_reg_to_grp1(reg, target, Operand::Byte(imm))
+                }
 
                 0x84 => modrmb!(self, TEST),
                 0x85 => modrmv!(self, TEST),