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Change JMPE encoding from being relative to being absolute

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Apparently NASM has been doing this in violation of the spec since
JMPE was first added. Hopefully fixing it won't cause new problems...

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
This commit is contained in:
H. Peter Anvin (Intel)
2025-10-12 12:58:04 -07:00
parent 44e89ba9b6
commit 0bc4333150
2 changed files with 34 additions and 4 deletions
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24
doc/changes.src
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@@ -11,6 +11,30 @@ evolving code simpler.
It is the production version of NASM since 2025.
\S{cl-3.02} Version 3.02
\b Fix build problems on C23 compilers using a pre-C23 version of
\c{<stdbool.h>} which defines \c{bool} as a macro in violation of the
C23 specification.
\b The immediate form of the \c{JMPE} instruction (opcode \c{0F B8})
has been changed to an absolute address, as in
the Itanium Architecture Software Developer's Manual, version 2.3,
Volume 4, page 4:249. Hopefully this won't break whatever virtual
environments use \c{JMPE}, but it is the closest thing there is to
an official specification for this opcode.
\> Being an \e{absolute} address, treat it equivalent to a \c{FAR}
jump and do not default to 64 bits in 64-bit mode.
\> That \c{JMPE} has apparently been wrong all these years is probably
as good of a hint as any how much it has been actually used, but it
\e{does} have the possibility of breaking virtual environments. In
that case, please file a bug report to \W{https://bugs.nasm.us/}
with details about the virtual environment, and we will figure out
a suitable solution.
\S{cl-3.01} Version 3.01
\b A new \c{obj2} version of the \c{obj} output format, intended for

14
x86/insns.dat
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@@ -307,10 +307,16 @@ $br $wdq LOOPZ near|short,cx# [i-: a# os e1 rel8] 8086,NOAPX
$br $wdq LOOPNZ near|short,cx# [i-: a# os e0 rel8] 8086,NOAPX
; JMPE is obsolete, but seems to be used by a fair number of virtual environments?
$br JMPE near [i: os 0f b8 rel] IA64
; 0f 00 /6 with a prefix has been repurposed in long mode
$wdq JMPE rm#|near [m: nw o# np 0f 00 /6] IA64,OSIZE
$wd JMPE rm#|near [m: o# 0f 00 /6] IA64,OSIZE,NOLONG
; However, REP-prefixed JMPE opcodes have been repurposed.
;
; Per version 2.3 of the Itanium Architecture Software Developer's Manual,
; which is the closest thing to an offical specification for JMPE,
; the JMPE instruction is absolute and takes an operand size prefix like
; a far JMP. As far JMPs are not promoted by default on 64 bits, follow that
; convention.
;
$wdq JMPE imm#|near [i: norep o# 0f b8 i##] IA64,OSIZE
$wdq JMPE rm#|near [m: norep o# 0f 00 /6] IA64,OSIZE
;# Call and return
$br CALL near [i: os e8 rel] 8086,BND,NOAPX