On 2017-10-13 10:37, Steve D'Aprano <steve+pyt...@pearwood.info> wrote: > On Fri, 13 Oct 2017 05:16 pm, Gregory Ewing wrote: >> Steve D'Aprano wrote: >>> On Fri, 13 Oct 2017 03:37 pm, Gregory Ewing wrote: >>> >>>>If the compiler can tell where p is initially pointing, it could >>>>put the pointer in read-only memory. >>> >>> If it's read-only, how can the compiler write to it? >>> >>> (I come from the days when ROM was actual ROM, burned in at the factory.) >> >> So, the factory is allowed to write to it. Possibly >> it's writing data that came from... a compiler? > > The data could come from anywhere, including a scanner: > > https://hackaday.com/2012/08/24/uncovering-easter-eggs-in-old-mac-roms/ > > I wasn't questioning where the data came from, but how the compiler can write > to READ ONLY MEMORY which might not even be in the same continent as the > compiler that generated the code.
A compiler rarely writes into the final destination memory (JIT compilers are the exception). It writes into a file. This file often has sections like "code", "read-only data", etc. Another tool (maybe the OS, maybe an EPROM burner) will later read this file and take appropriate actions. "the compiler could put the pointer in read-only memory" is just shorthand for "the compile could put the pointer into one of the read-only sections of the object file". > Read-only memory (ROM) is typically burned into the silicon by the integrated > chip manufacturer at the factory: > > https://en.wikipedia.org/wiki/Mask_ROM In this case there will be tool (or more likely a whole tool-chain) which takes the read-only sections of the object file and converts them into a lithographic mask which will then be used to create chips. The pointer will end up as a bunch of transistors. > or written by a dedicated hardware device: > > https://en.wikipedia.org/wiki/Programmable_read-only_memory And in this case there will be a tool which will read the object file and send the contents of the read-only sections to the device which writes the ((E)E)PROM. And finally, the most frequent case: The OS will will read the executable into RAM, mark those pages from the read-only sections as read-only in the page table and start it. > Whether the data is burned into the silicon or electrically written by a > dedicated device, it isn't written by the compiler, and once written the data > is permanent. Did anyone claim that? >> A memory that couldn't be written to at all, ever, would be a bit >> useless! > > Macs used a ROM for at least a decade and probably more. The ROM contained > data such as mouse cursors, toolbox routines, icons, sounds, and a > bootloader. No Mac was capable of writing to their ROMs any more than they > could write to their mouse or a CD-ROM. Did anyone claim that? hp -- _ | Peter J. Holzer | Fluch der elektronischen Textverarbeitung: |_|_) | | Man feilt solange an seinen Text um, bis | | | h...@hjp.at | die Satzbestandteile des Satzes nicht mehr __/ | http://www.hjp.at/ | zusammenpaßt. -- Ralph Babel -- https://mail.python.org/mailman/listinfo/python-list
