Memory protection

• V.1 Four types of memory
• V.2 PRG flags for protection
• V.3 Mxalloc() protection flags

V.1 Four types of memory

Memory protection works on a per-process basis. Unless it uses the special mode bits in Mxalloc(), a process' entire text+data+bss and any memory it allocates will be of the same type. There are four types of memory:

By default, all processes load into private memory. If a program terminates and remains resident with the Ptermres() system call, then its private memory will be converted to Super memory; that's because TSRs won't work in private memory. Some TSRs may in fact need to be set to load into global memory (if they provide services for user mode programs).

There are other programs which need to grant wider access to their memory space: for example, the Gulam CLI is one, because Gulam's children can call Gulam through a pointer found in the system variable space. But Gulam is a bad choice of shell anyway, because you can't run more than one of them. Current versions of STeno also must grant wider access, because it passes data to STalker using shared memory; private/readable is adequate for that. Programs that load before MiNT does (i.e. in the AUTO folder) will find themselves in global memory. This is the easiest way to be sure that your TSRs will go on working: RAMdisks, FOLDR100, CACHE080, etc. all work when loaded that way. Notice that a process can grant wider access to its own address space, but it can't change the access permissions of any other process' space. If you set the PRGFLAGS of some process so it loads into global memory, that doesn't mean it can access anybody else's memory, it just means that anybody can access its memory.

V.2 PRG flags for protection

The second-least-significant nybble of PRGFLAGS field in the program header is the default mode for your program's memory protection. To review, here are all the defined bits in the PRGFLAGS field of PRG file headers, and what they mean:

You can use prgflags.prg to change the PRGFLAGS value for any program. From the shell, say (for example)

prgflags 0xf0 0x10 myprog.prg

to set myprog.prg to global (0x10). The way you use PRGFLAGS is to give it two numbers, then the program name(s) to operate on. The first number is taken as a mask, and the second as a value: bits which are one in the mask will be set to the corresponding bits in the value; bits which are zero in the mask will not be changed. Thus the example sets bits 4-7 to 0001 and leaves all other bits alone.

V.3 Mxalloc() protection flags

You can specify the protection you want on a region when you Malloc it: call Mxalloc(size,mode), where the least- significant nybble of mode means what it always has, and the next four bits are the desired protection mode.