5 ms·

Undocumented 8086 Opcodes, Part I

56 points by beefhash 9y ago

Sniffnoy 9y ago
Hm, seems to just be part 1 so far... Edit: This seems to be an earlier, less detailed post on the same subject, but which does mention one or two things this post doesn't: http://www.os2museum.com/wp/undocumented-8086-opcodes/ http://www.os2museum.com/wp/undocumented-8086-opcodes/
exikyut 9y ago
Can someone tell me why almost every time I visit this site I get 403-blocked? Very occasionally it works, but only very occasionally. This is the first time I've loaded the site from a HN story (most visits were from searches); maybe it's overload, it might not be.
- scruffyherder 9y ago
  I get it from my home all the time, but it works from the office, or mobile phone....
- lathiat 9y ago
  Blocked for me too.. I'm coming from Australia on my home static IP. Weird.
  - exikyut 9y ago
    I'm in Australia too, incidentally. Interesting.
tinus_hn 9y ago
If you’re interested in this, check out the Visual 6502 project that shows where these codes come from, but on the 6502. Unfortunately they don’t have good enough die shots for the 8086 yet. http://visual6502.org http://visual6502.org
jakear 9y ago
Could anyone explain why POP CS is so "obviously useless"? The author says it is "as obvious as it is useless", and never explains further. As far as I can understand, CS is the top byte of the 16-bit instruction pointer, so sure you wouldn't want to go around changing just that byte. But I feel like I'm missing something because that doesn't seem too crazy out of this world useless to get such a special mention.
- cdumler 9y ago
  Execution occurs with two registers, Code Segment and Instruction Pointer. Normally, you use other operation codes (JMP, CALL, etc) because they will take into account modifying IP or CS:IP, as appropriate. It would be a really odd situation where you would change the CS, not update IP, and have the proper alignment where it can execute the next instruction. Normally, all of that is computed by the compiler/linker and executed with proper operation codes. The 8086 is a weird beast where the memory access is two 16 bit values mapped to 20 bits (ie. 1 megabyte into overlapping 64k blocks offset by 16 bytes each time). It was easier to write code staying inside a single segment, so chunks of code were often written staying inside one. Nothing particularly useful comes to mind just updating CS; but, my assembly days were a long time ago.

56 points by beefhash 9y ago

Hm, seems to just be part 1 so far... Edit: This seems to be an earlier, less detailed post on the same subject, but which does mention one or two things this post doesn't: http://www.os2museum.com/wp/undocumented-8086-opcodes/ http://www.os2museum.com/wp/undocumented-8086-opcodes/

exikyut 9y ago

Can someone tell me why almost every time I visit this site I get 403-blocked? Very occasionally it works, but only very occasionally. This is the first time I've loaded the site from a HN story (most visits were from searches); maybe it's overload, it might not be.

scruffyherder 9y ago
I get it from my home all the time, but it works from the office, or mobile phone....
lathiat 9y ago
Blocked for me too.. I'm coming from Australia on my home static IP. Weird.
- exikyut 9y ago
  I'm in Australia too, incidentally. Interesting.

tinus_hn 9y ago

If you’re interested in this, check out the Visual 6502 project that shows where these codes come from, but on the 6502. Unfortunately they don’t have good enough die shots for the 8086 yet. http://visual6502.org http://visual6502.org

jakear 9y ago

Could anyone explain why POP CS is so "obviously useless"? The author says it is "as obvious as it is useless", and never explains further. As far as I can understand, CS is the top byte of the 16-bit instruction pointer, so sure you wouldn't want to go around changing just that byte. But I feel like I'm missing something because that doesn't seem too crazy out of this world useless to get such a special mention.

cdumler 9y ago
Execution occurs with two registers, Code Segment and Instruction Pointer. Normally, you use other operation codes (JMP, CALL, etc) because they will take into account modifying IP or CS:IP, as appropriate. It would be a really odd situation where you would change the CS, not update IP, and have the proper alignment where it can execute the next instruction. Normally, all of that is computed by the compiler/linker and executed with proper operation codes. The 8086 is a weird beast where the memory access is two 16 bit values mapped to 20 bits (ie. 1 megabyte into overlapping 64k blocks offset by 16 bytes each time). It was easier to write code staying inside a single segment, so chunks of code were often written staying inside one. Nothing particularly useful comes to mind just updating CS; but, my assembly days were a long time ago.