assembly - x86 32 bit opcodes that differ in x86-x64 or entirely removed -


i've looked wikipedia x86 backward compatibility in x86-x64 , says:

x86-64 backwards compatible 16-bit , 32-bit x86 code.because full x86 16-bit , 32-bit instruction sets remain implemented in hardware without intervening emulation, existing x86 executables run no compatibility or performance penalties,whereas existing applications recoded take advantage of new features of processor design may achieve performance improvements.

so i've tested instructions see yield entirely different opcodes (rather applying prefix) such as: inc/dec. looking @ (x86):

\x40 inc eax
\x48 dec eax

and while assembling same in x86-x64 yields:

\xff \xc0 inc eax

\xff \xc8 dec eax

i'm trying figure out reason , more examples of other instructions has same symptoms yield different opcodes. familiar push, pop, call, ret, enter , leave not available 32 bit in x86-x64.

the instructions entirely removed obscure obsolete ones, binary-coded-decimal stuff , aam (ascii-adjust after multiplication) fixing binary-coded-decimal after doing normal binary add/sub/mul/div.

however, encodings of instructions removed: in case, 32bit can use inc r32 single-byte opcodes (0x40 + register-number). 64bit mode has inc r/m32 encoding, register incremented specified 2nd byte. (in case, 0x4x bytes repurposed rex prefix byte).

intel's insn reference (follow link in https://stackoverflow.com/tags/x86/info) shows following inc:

opcode   instruction op/   64-bit   compat/                      en     mode    leg mode   ff /0   inc r/m32     m     valid     valid      increment r/m doubleword 1.  40+ rd  inc r32       o      n.e.     valid      increment doubleword register 1.

n.e. means not encodable.

the op/en column describes how operands encoded.

jan hubicka's amd64 isa overview briefly describes repurposing of single-byte inc/dec opcodes rex prefixes, , default operand sizes , how immediate data still 32bit. movabs available loading 64bit immediate constants, or load/store from/to 64bit absolute address.

amd's amd64 manual, section 2.5.11 reassigned opcodes has table quite short. lists:

  • 4x inc/dec r32 turned rex prefixes
  • 63 arpl became movsxd (sign-extend dword qword, when used rex.w=1 (which means w bit in rex prefix = 1)).

early amd64 , intel emt64 cpus left out sahf/lahf in long mode, later re-added instruction same opcode in 32bit. table doesn't list instructions removed entirely (the bcd instructions , maybe others) removed make room possible future extensions.

they have simplified things lot, , made x86-64 better cleaner instruction set more room future extensions, every difference 32bit means more decoder transistors. there no machine instructions moved different opcode in 64bit.

multiple machine instructions share same asm mnemonic, mov being overloaded one. there loads, stores, mov immediate-constants, move to/from segment registers, in 8bit , 32bit. (16bit 32bit operand-size prefix, same 64bit rex prefix.) there's sepecial opcode loading rax 64bit absolute address. there's special opcode loading 64bit immediate-constant register. (at&t syntax calls movabs, it's still mov in intel/nasm)


Comments

Post a Comment

Popular posts from this blog

html - Outlook 2010 Anchor (url/address/link) -

i'm trying link specific pdf page msoutlook'10 or msword'10, #page=... anchor particularly useful , works fine elsewhere. unfortunately when click [ctr-k] in outlook or winword, links re-formatted assuming unc path. pdf file located on shared drive. using "file:" prefix links open pdf in adobe reader (outside of browser), , "http:" open inside browser. issue seem need open in browser "file:" prefix in order anchor work. know anchors work inside browser. outlook uses windows/explorer default program (based on file extension) seems open in adobe reader neglecting anchor. is aware of solution, link specific page or bookmark of shared-pdf ms outlook and/or winword link? common references: mailermailer.com/...anchor-tags-html-emails campaignmonitor.com/...anchor-links-in-email-newsletters/ stackoverflow.com/...anchor-in-pdf-document
Read more

javascript - Why does running this loop 9 times take 100x longer than running it 8 times? -

consider code: test = function() { } t = new test(); (var = 0; < 8; i++) { result = t instanceof test; } if change number of iterations 8 9 , loop take 100 times longer complete in latest version of firefox (41.0.1). tested on 2 different pcs , magic limit 8. here jsperf test used: http://jsperf.com/instanceof-8-times-vs-9-times does have idea why might happen? seems specific instanceof . not happen if else object, example check property. note: filed bugzilla bug this. jan de mooij mozilla team has posted details in bugzilla thread . here's simplistic interpretation of highly technical answers: in i < 8 case, firefox smart enough hoist result = t instanceof test; statement out of loop (according tests, doesn't seem omit altogether). in i < 9 case, apparently doesn't optimization . why? reason not clear, related fact 9 iterations threshold above function considered "hot" enough run through jit compiler. i < 8 cas...
Read more

Getting gateway time-out Rails app with Nginx + Puma running on Digital Ocean -

following this guide setup app on digital ocean droplet, not working. site not loading , it's throwing: "gateway time-out". the puma server works, since ps aux | grep puma prints: root 15878 0.0 0.4 376832 2168 ? sl 12:50 0:00 puma 2.13.4 (unix:///var/www/myapp/shared/tmp/sockets/myapp-puma.sock) [20150930164940] root 15881 0.0 0.7 903676 3692 ? sl 12:50 0:06 puma: cluster worker 0: 15878 [20150930164940] root 24654 0.0 0.1 11748 916 pts/2 s+ 18:00 0:00 grep --color=auto puma my nginx config: user root; worker_processes 1; events { worker_connections 1024; } http { upstream puma { server unix:///var/www/myapp/shared/tmp/sockets/myapp-puma.sock; } server { listen 80; server_name myapp.com; root /var/www/myapp/current/public; access_log /var/www/myapp/current/log/access.log; error_log /...
Read more