cpu.x86.assembler: add segment override prefixes

basis/cpu/x86/assembler/assembler-tests.factor

@@ -164,3 +164,5 @@ IN: cpu.x86.assembler.tests
 
 [ { 15 183 195 } ] [ [ EAX BX MOVZX ] { } make ] unit-test
 
+[ { 100 199 5 0 0 0 0 123 0 0 0 } ] [ [ 0 [] FS 123 MOV ] { } make ] unit-test
+

basis/cpu/x86/assembler/assembler.factor

@@ -188,6 +188,13 @@ M: register displacement, drop ;
 
 PRIVATE>
 
+! Segment override prefixes
+: CS ( -- ) HEX: 2e , ;
+: ES ( -- ) HEX: 26 , ;
+: SS ( -- ) HEX: 36 , ;
+: FS ( -- ) HEX: 64 , ;
+: GS ( -- ) HEX: 65 , ;
+
 ! Moving stuff
 GENERIC: PUSH ( op -- )
 M: register PUSH f HEX: 50 short-operand ;