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> A MOV instruction is not the same size on all platforms
That’s why you tell the compiler which architecture you’re targeting, I think I get it.
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It's possible to do. GCC is like that where it is a kind of modular approach. You can specify a platform you're compiling for and it'll use the appropriate compiler "module" to target the platform you specify.
An assembler can be written the same way, targeting a certain chipset, but even then, there are differences inside the same chip, called a "stepping" in modern parlance, where there are bug fixes in the hardware that an assembler has to take into account to generate correct code.
For example, the original 1975 production 6502 CPU had three shift/rotate instructions, ASL, LSR, and ROR. The ROR instruction was bugged where it didn't work correctly[^]. If the assembler didn't know about the bug and generated code that assumed the instruction was correct, your code would probably crash the system. The assembler had to know about the bug and generate equivalent code that didn't use the instruction.
The bug was fixed starting with the 1976 production.
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Dave Kreskowiak wrote: The bug was fixed starting with the 1976 production.
Sidetracking just a little bit, but I can't resist  :
At CERN, the same thing happened with two completely independent computer families: First, CERN had bought the VAX 780, the very first VAX. When DEC announced the smaller VAX 750, they proudly announced that the 780 bug that made one instruction give rather unexpected results (sorry, I don't remember which instruction), CERN immediately stood up in protest: No! We have written our software to specifically compensate for that bug! If you change it in the 750, so that it gives a different, 'correct' result, we must update our software for that, and we have to maintain different program versions for the 780 and the 750. That is out of the question, and we will not buy any VAX 750.
The bug was not fixed in the VAX 750.
And the story repeats: CERN had used NORD-10 computers for process control (much due to extremely good interrupt handling: The first instruction of the handler was running 900 ns after the arrival of the interrupt signal, which was super-fast in the mid-1970s). Then comes a complete reimplementation of the architecture, labeled ND-100, with a similar happy message: Finally, we have fixed the bug that has been with the NORD-10 since its introduction ... CERN reacted in the same way: If you fix that bug, we can no longer run our software on your new machines; we have adapted to that bug! We will not buy any ND-100!
So the bug from NORD-10 was retained in the ND-100 - just like with the VAX.
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Usually, a machine instruction is a word of, say, 32 bits. The first 6 to 8 bits (typically) tell what this instruction does: Move data, jump to somewhere, add two values etc. The next few bits may indicate how you go about finding the operand, i.e. what to move, where to jump or which value to add. The interpretation of the following bits depend on those (often called the 'address mode' bits): Either as a register number, how far to jump, or the value to add. If the address mode bits so says, maybe the value to add is not in the instruction itself, but the instruction tells in which register you can find the address of the value to add.
The compiler breaks down your code in more primitive operations, such as "add the value of X", without being concerned about what the proper machine instruction will look like. Not until it gets down to the very bottom, the 'code generator'. A compiler may provide different code generators for different machine types. A given code generator knows what an 'add' instruction looks like on x64 processors, knows the proper address mode bits and how to put the address of X into the instruction. Another code generator, for ARM, knows another code for ADD on the ARM, but it also knows that you cannot directly add something in memory. First the code generator must look up an unused register (and if there is none, it must generate an instruction to flush the value in one register back to memory to free it up), then generate an instruction to load X into the free register, and then generate an instruction to do the actual add of the newly loaded value.
There is no common, standardized code for neither move, jump nor add on different machines. The code generator knows the codes for this machine. You may tell the compiler to switch to another code generator knowing the codes for another machine (that is commonly referred to as 'cross compiling'), but the program that comes out of it cannot be run on this machine; you must copy it to another machine of the right type to run it.
In the good old days, there were dozens of different machine types out there, each with its own instruction codes. The last 35 years or so, the 'x86' has pushed most others out. Every PC in the world can understand x86 codes.
But x86 is for 32 bit machines! 64-bit PCs understands 'x64' codes (and address modes), which are different! If you want that move, jump or add to work on a 64-bit PC, your compiler must select a code generator knowing the proper codes for x64. The program will not work on an old x86 PC.
Relax ... The x64 CPUs can be told to forget the x64 codes, and run the x86 instead. The .exe file tells which instruction set should be activated, so you can run 35 year old programs on your brand new 64 bits PC (provided that your current OS will honor all the requests made by that old program, which is not guaranteed - but 20-25 years is probably on the safe side).
Then, what about your smartphone - will it know x64 move, jump or add instruction codes? No. Will it know x86 instruction codes? No. You have to ask your compiler to use the code generator that makes ARM instruction codes. ARM has 32 bit and 64 bit codes too, that are different ... Besides: If your program was written for Windows, it expects that is can ask the OS (i.e. Windows) for this and that service - and Android says Huh?? The services provided by Android are quite different.
Bottom line: The rosy, cosy days when x86 worked everywhere are over.
Then comes dotNet. When a dotNet 'assembly' (informally you may call it a module) is loaded into your machine, smartphone or whatever, you'll see an incompletely compiled program: The compiler has left a message: Here I should have generated the code for adding the value of X, but I didn't know what kind of machine this will be running on! So please, before running this program, generate the proper instructions for adding X, will you?
dotNet for a given machine has the proper code generator for that machine. dotNet on an ARM32 generates ARM32 codes, dotNet on an ARM64 generates ARM64 codes, dotNet on a 64 bit PC generates x64 codes. They are all different.
For now, Windows itself is not dotNet, so it must be compiled separately for every machine architecture. A growing number of applications are dotNet, incompletely compiled, and the last step of compilation, code generation, is not done until you know for sure which codes to use, just in time for execution. So the dotNet code generator is frequently referred to as the "just in time compiler", or "jitter".
ps.
If you want to look at instruction codes and addressing modes and such to see what they are like, my recommendation is to stay away from x64 and x86. They are both a mess, having grown and been extended and grown more and been extend ... into a crow's nest. ARM64 (aka. Aarch64) is certainly not the simple, easily understood thing that the early 32 bit ARMs were, yet it has retained a much more manageable structure.
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tronderen and Dave, thanks for your extensive explanations. At this point I don’t understand all the bits but I think I’m closing in
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All you really need to understand is:
- a compiler can be written in anything (more or less) from machine code, through assembler up to most high level languages.
- The output of the compiler must be code that is compatible with the machine that will run the final executable.
- the term "machine" can be the actual hardware, a virtual machine (like the Java Virtual Machine), or Framework such as .NET.
- the actual hardware instructions do not have to be the same across all platforms, but it would be nice. Just as USB connectors keep changing so hardware platforms keep evolving.
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You can write your own compiler.
Simplest way is to do it, without doing must studying, is to write a 'calculator' which takes tokens like numbers and the plus sign and equals sign.
After you do that then read up a bit more on compilers and apply some of what you read to what you previously wrote.
Then add variables.
If you really want to keep going after that then you add 'if-then-else' because that structure has problems of which compiler theory talks about quite a bit.
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The very short version...
In the beginning there was machine language very closely tied to the CPU. It worked with numeric opcodes that programmers has to literally memorize or look up. This got old real quick.
ML Pseudocode: Instruction Operation
00000000 Stop Program
00000001 Turn bulb fully on
00000010 Turn bulb fully off
00000100 Dim bulb by 10%
00001000 Brighten bulb by 10%
00010000 If bulb is fully on, skip over next instruction
00100000 If bulb is fully off, skip over next instruction
01000000 Go to start of program (address 0)
Enter assembly. It's not a compiler. It's an assembler and also a linker as part of a toolset. There's a difference. A compiler will translate code into something that's a one-to-one correlation with machine instructions. Assembly is already that. It's a language that basically gave human-memorable mnemonics to the opcodes. It was originally written in machine language. It's very CPU specific too. This too got old.
There were a ton of other languages made, presumably written in ASM, but this is where a compiler kicks in. To make a really long story short, I'll just mention C's history. C was based on B and B was based on BCPL. I don't know what BCPL was written in, but the first B compiler was written in BCPL. Eventually, the B compiler was re-written in B itself and then the first C compiler was written in that version of B.
A language written its own compiler happens more than you'd think. Anyway, these are still native compilers and eventually they still make their own down to machine code. Now, things like Java and C# I suspect are still written in native languages for obvious reasons, but don't be surprised if a native language's compiler is written in the same language.
Calin Negru wrote: a standard should be required where the numbers/machine instructions for MOV are recognized everywhere. I mean it should work like a hardware resource with the same ID present on old and new processors.
This sounds great in theory, but if you look at how bloated and not-fun the Win32 API is, if you always have to maintain backwards compatibility then you keep things bloated when attempting to advance. I mean, it's good on one level, but it's also good to wipe out the old and try something new, like Apple is doing with the M1 chips (even though nothing is every really new, but you get the idea).
Do we really want processors in 100 years having similar constraints as one designed in the 1960s? Rather than enforce that on the CPU, the industry has (correctly so) to rather have compiler targets implemented. You use your preferred language and it compiles down to whatever the CPU expects with optimizations, etc.
Jeremy Falcon
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Jeremy Falcon wrote: C was based on B and B was based on BCPL. I don't know what BCPL was written in, but the first B compiler was written in BCPL
Whilst I, too, don't know what BCPL was written in, I did hear why it was called BCPL. WikiPedia[^] says it stands for "Basic Combined Programming Language" and was invented in Cambridge University (UK). The story that I heard was there was a more complex language jointly designed by universities in Cambridge and London - that was call CPL (Cambridge Plus London). I do not know if CPL saw the light of day; but a simplified version called Basic CPL (or just BCPL) was created.
It had a bizarre construct, which was definitely a candidate for CPs Wierd and Wonderful forum), to resolve the 'Dangling ELSE problem'. It was something like IF condition DO statement and TEST condition THEN statement OR statement. (See https://www.bell-labs.com/usr/dmr/www/bcpl.pdf[^])
Edit:
I've just read the Wikipedia article (I should have done that before posting!). It says the CPL language was named originally from 'Cambridge Programming Language' and later renamed to 'Combined Programming Language'. No mention of London. But CPL (programming language) - Wikipedia[^] does mention the involvement of London and it was nicknamed 'Cambridge Plus London'. Thus, the name I heard was not its real name.
modified 7-Feb-23 5:26am.
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That's cool to know. Thanks for sharing. 
Jeremy Falcon
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Message Closed
modified 15-May-23 19:06pm.
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Where did you find this expression?
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Did you read the documentation[^]?
"In testa che avete, Signor di Ceprano?"
-- Rigoletto
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Maybe if you actually provided the full details ...
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[feb 2,2023,3:01am] same as Victor, could you describe the context of that message? A veteran probably needs no further clue to guess the source of where that came from but some of us are not veterans (not me at least)
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Because you're too lazy to do the work yourself, you post this nonsense just so you can down vote answers and legitimate questions? You are a troll.
Quote: From the perspective of an application, a "cancellation point" is any one of a number of POSIX function calls such as open(), and read(), and many others.
"Before entering on an understanding, I have meditated for a long time, and have foreseen what might happen. It is not genius which reveals to me suddenly, secretly, what I have to say or to do in a circumstance unexpected by other people; it is reflection, it is meditation." - Napoleon I
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Gerry Schmitz wrote: You are a troll.
I tend to agree; he certainly has history. Also complains of being thrown out of other forums for "asking too many questions", but I suspect the real reason is not that.
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Hi
just had some discussions. on IBMMAIN regarding the C++ code (DLL) using C++ I developed and tried porting to z/os
since I need a lot of the same functionality
I have been compiling on Z/OS XL C++ and I got some differences
as an example
auto ret = procpointer->extsymcollector->insert({ *s, *exsympointer });
where the XL C++ compiler didnt like the '{'
I was told by someone who works on the XL C++ compiler to ditch MSVC
and go with CLNG/LLMV
by going here
Clang/LLVM support in Visual Studio projects | Microsoft Learn[^]
As MSVC only goes to C++ 11
in addition I was told to ditch XL C++ and go to Open XL C++ As that goes to C++ 17 or 18 and is baseD on CLANG/LLVM
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ForNow wrote: As MSVC only goes to C++ 11
No, it fully supports C++20.
Go to project properties pages -> General -> C++ Language Standard and select "ISO C++ 20 Standard".
Now, for that particular piece of code, the "{}" is the C++ initialization syntax available since C++11. You can try replacing that with:
auto ret = procpointer->extsymcollector->insert(T(*s, *exsympointer)); where T is the type of object that is inserted.
Mircea
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David Crayford who works on the XL C\C++ z/os compiler suggested I switch my compiler from MSVC to CLANG\LLVM for a few reason one then seems to be easier portability
What’s your opinion
Thanks
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I'm a MSVC and Visual Studio fan. I find it a superb development environment. Compiler is just one piece of the puzzle, but you also need a good editor and a good debugger. All in all, for day to day development, I think Visual Studio is hard to beat. More than once, after developing in Visual Studio I had to port to g++ and I never had any major problems.
Mircea
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