duskos

commit bd9afb80910e4d9fff7a6b687cb0f214be80ef03
parent dd04bc4bc2b35f03229a4f28144cd55272f9dc0c
Author: Virgil Dupras <hsoft@hardcoded.net>
Date:   Sun, 30 Oct 2022 16:47:44 -0400

asm/i386: add two shortcut encoding forms

1. The "AX + imm" short form
2. The "rm/32-or-16 + sign-extended-imm8" short form

Diffstat:
M
fs/asm/i386.fs
 | 
15
++++++++++-----

1 file changed, 10 insertions(+), 5 deletions(-)
diff --git a/fs/asm/i386.fs b/fs/asm/i386.fs
@@ -59,7 +59,7 @@
   op16b if $66 c, then dup 8 rshift $ff and ?dup if c, then c, ;
 
 : modrm, ( -- ) \ write down modrm, errors out if not all parts are there.
-  opmod 3 lshift opreg or 3 lshift oprm or dup $100 < _assert c, ;
+  opmod 3 lshift opreg or 3 lshift oprm or dup isbyte? _assert c, ;
 
 : sib, ( -- ) \ write down the SIB byte if needed.
   realmode not if oprm SIB = opmod 3 < and if sib c, then then ;
@@ -80,7 +80,9 @@
   opdirec << or op, msd, asm$ ;
 
 : opimm, ( opcode opreg -- ) \ write the operation in "immediate" mode
-  opreg! maybe8b op, msd, imm, asm$ ;
+  opreg! op8b not if
+    1 or imm isbyte? if ( sign-extended imm8 ) 8b! 2 or then then
+  op, msd, imm, asm$ ;
 
 \ Setting arguments
 
@@ -174,7 +176,7 @@ $9c op pushf,      $9d op popf,        $cf op iret,
 \ of the size of its relative offset. To that end, we auto adjust that relative
 \ address to the size of the op. Therefore, "0 jmp," is an infinite loop encoded
 \ as EB FE.
-: jrel8? ( rel -- f ) $7f + $100 < ;
+: jrel8? ( rel -- f ) $7f + isbyte? ;
 : jrel8, 2 - c, ;
 : jrel32, ( rel32-or-16 ) realmode if 3 - w, else 5 - , then ;
 
@@ -218,7 +220,7 @@ $e2 op loop,     $e1 op loopz,   $e0 op loopnz,
 \ r = opreg override
 \ m = modrm opcode
 : op ( reg opcode -- ) doer , does> @ ( opcode -- )
-  dup 16 rshift opreg! $ffff and dup $100 < if maybe8b then opmodrm, ;
+  dup 16 rshift opreg! $ffff and dup isbyte? if maybe8b then opmodrm, ;
 $0400f7 op mul,    $0600f7 op div,     $0300f7 op neg,     $0200f7 op not,
 $0100fe op dec,    $0000fe op inc,
 $000f9f op setg,   $000f9c op setl,    $000f97 op seta,   $000f92 op setb,
@@ -234,7 +236,10 @@ $020f01 op lgdt,   $030f01 op lidt,
 \ m = modrm opcode
 : op ( opcode -- ) doer , does> @ ( opcode )
   imm? if
-    8 rshift dup >> $fc and $80 or swap 7 and ( opcode opreg ) opimm,
+    opmod 3 = opreg AX = and if \ AX shortcuts
+      16 rshift maybe8b op, imm, asm$
+    else
+      8 rshift dup >> $fc and $80 or swap 7 and ( opcode opreg ) opimm, then
   else $ff and maybe8b opmodrm, then ;
 $040000 op add,    $3c0738 op cmp,     $2c0528 op sub,     $a80084 op _test,
 $240420 op and,    $0c0108 op or,      $340630 op xor,     $000086 op xchg,