In modern operating systems (for example: Windows), you can't access a memory location before you allocate that memory location to your program (or else a segmentation fault will occur).
I am wondering, do you need to also allocate memory before you use it in MS-DOS?
The basic answer is “yes”, assuming you want to be a good DOS citizen, however in many cases you don’t need to worry about it because the operating system takes care of it for you.
If you’re talking about conventional memory, allocating it explicitly isn’t necessary:
DOS does provide APIs to allocate memory blocks, resize them and free them: respectively, interrupt 0x21, function 0x48, interrupt 0x21, function 0x4A and interrupt 0x21, function 0x49. Function 0x58 affects how memory is allocated, including whether UMBs are considered (in MS-DOS 5 and later, and compatible implementations).
Some programs need to allocate memory separately, and therefore use these functions. One example is TSRs, since they need to reduce their pre-allocated block as much as possible and perhaps allocate other memory blocks, in order for them to remain allocated after they return control to the operating system but use as little as possible. It can also be useful to allocate memory if you want to use all the available memory: memory fragmentation might mean you wouldn’t get all the available memory in your pre-allocation.
You’d need to explicitly manage memory allocations if you wanted to use anything outside conventional memory: upper memory blocks, the HMA, EMS, XMS or plain extended memory.
For the sake of offering a contrary answer: there is no memory protection and there are no access levels, so there will be no segmentation fault or other hardware interrupt if you access memory that is there and which you have not asked for ownership of. If you are an MS-DOS program then you are driving the computer. Various of the interrupts will be set up to route through DOS when your program starts, but there is no access control to prevent you from disabling or rerouting every single one of them.
So you don't need to request ownership of memory provided that:
In practice you probably don't know those things. So instead of your code being declared faulty and evicted without harming the system as it would under a modern OS, you'll probably continue to run until you've broken some other part of the system and that part tries to run. E.g. maybe the user has a Gravis Ultrasound emulating a Sound Blaster, or is using MSCDEX plus their particular CD driver, or has their manufacturer's VESA driver occupying a certain block of memory, you stomp on that storage, then attempt to output audio, read from the CD or change the graphics mode.
I think that as to your question, the answer is therefore both yes and no. You don't need to allocate memory to be able to use it, but if you don't then you accept the risk of causing some other part of the system to crash or otherwise misbehave.
It's up to you though.
Technically, no. You DO NOT absolutely have to allocate memory before you use it in MS-DOS (for versions prior to the DOS that was included in OS/2 or the one in Windows 3.0). But note also that the ability to access any memory space (or not) in a machine is also determined by the hardware architecture (i.e., 8088, 8086, 80286, 80386, etc.) Now, to focus solely on the responsibility of the operating system (OS) here, Robert J. Moore wrote in a November 01, 1988 article on Dr. Dobbs that, "Starting with Version 2.0, all versions of DOS contain functions to allocate memory (function 48H), free allocated memory (function 49H), and modify allocated memory (function 4AH)," via the use of Memory Control Blocks (MCBs). "If DOS detects any corruption in the MCB chain, it prints out an error message and halts the system." However, it is possible to, "break the MCB chain deliberately . . . DOS doesn't always notice." In fact, "DOS could be bypassed altogether for memory management," said Mr. Moore. Simply put, that's because BOTH the OS and the hardware allows it. Per the book, Intel Microprocessors: 8086/8088 - 8th Edition (available on Google Books), "Real mode, is an operating mode of 80286 and later x86-compatible CPUs. . . Real mode provides no support for memory protection, multitasking, or code privilege levels. 80186 CPUs and earlier, back to the original 8086, have only one operational mode, which is equivalent to real mode in later chips." Now, protected mode, as opposed to real mode, first available on the 80286, could allow your DOS programs to come under protected mode memory management--but that can happen only if you've installed OS/2 or Windows 3.0 (which are the first operating systems to implement versions of DOS capable of DPMI--the protected mode specification) which would allow your DOS programs to run in protected mode on 80286 series and later processors. Only then would you actually need to allocate memory before using it in your DOS programs.
