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macOS Catalina runs only on 64-bit, so when I reload my existing Microsoft Office files, will they be "read only"?

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The 32-bit/64-bit description refers to executable code, rather than data. Applications and other executable code will contain instructions that are either 32 or 64 bits wide.

However, file formats are largely unaffected by this. Almost all applications will open the same file data regardless of the application's bit-depth. So, MS Office documents can be opened by both 32 and 64-bit versions.

It's possible that new software may introduce new data structures than are unreadable by older software, but that's a different issue.

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    Nitpick: "executable code will contain instructions that are either 32 or 64 bits wide." That's not actually the case; 32/64 bits is the word size that defines the width of the registers and data pipelines which the instructions operate on, which is not necessarily the same as the instruction length. (x86 and x86_64 both use a variable-width instruction encoding between 8 and 120 bits, while both arm32 and arm64 have a 32-bit fixed-width instruction set.) But that's just pedantry, and this is a great answer. – NobodyNada Jun 12 at 3:04
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    A way a format could be affected would be (badly) serialized data from a 32-bit program. But I'd consider this a bug in the software, however I have happened to stumble upon such applications at times. – ljrk Jun 12 at 8:46
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    @NobodyNada if you want pedantry, then don't forget that Thumb instructions are 16-bit wide, and Thumb-2 ones are 16- or 32-bit, depending on the instruction. – Ruslan Jun 12 at 10:35
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    @NobodyNada: x86 machine code is a byte stream not a bit stream, with instructions always aligned to byte boundaries. I've never seen anyone describe it as 8 to 120 bits, instead of 1 to 15 bytes, because the former doesn't exclude the possibility of 9-bit instructions for example. But yes, i386 and x86-64 are two different architectures that use incompatible machine code. Most of the opcodes are the same, but x86-64 changed the meaning of some to be a prefix for more registers, and for signalling 64-bit operand size. And changed the default operand-size of stuff like push/pop. – Peter Cordes Jun 12 at 12:25
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Files themselves don't have a bit-depth, as such, only applications.

If you have a current MS Office installed [ie not Office 2011 or earlier*], your files will be fine. If not, there are other apps that can handle them.
*Office 2016 is Catalina-compatible, if it is up to date.

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  • Ever seen a spreadsheet > 4GB? – Joshua Jun 12 at 1:34
  • The spreadsheet size is limited by the file format, not the word size of the processor running Office. – Thorbjørn Ravn Andersen Jun 12 at 10:27
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No. Macs have been, hardware-wise, 64-bit for a long time and even if Mojave technically supports 32-bit apps, most recent apps including Microsoft Office are 64-bit.

Even if it wasn't, things like text files and word files and spreadsheets and etc. aren't specific to 32-bit or 64-bit systems. Editing a file on a 32-bit system won't make it "read only" on a 64-bit system. Even if you were upgrading your system from 32-bit to 64-bit, your documents would remain the same.

Perhaps if you were weird and using some 32-bit office suite, you would have to upgrade the suite itself in order to start the program that can read those files, but the files themselves will be fine.

32-bit and 64-bit only refers to executable programs, specifically the type of instructions the CPU understands. Anything that is not machine code does not care about 32-bit or 64-bit - that includes documents.

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There is a very good Apple suite of tools that are compatible. It used to be called iWork but now it just each individual apps. They are Pages, Numbers, and Keynote. These can replace Word, Excel and Powerpoint and read / write those formats.

If you’re worried any third party app won’t work with the new OS - try the alternatives so you know how you like them.

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  • I don't see how this addresses the question. The OP wants to know whether opening old files in a 64-bit app will cause problems. – benwiggy Jun 12 at 7:43
  • The question got edited from when I answered it. It wasn't as obvious as it is now! – jmh Jun 12 at 15:05

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