Does using FreeDOS allow my program to access more than 64 K of memory?

Question

I am interested in programming in C on FreeDOS while learning some basic ASM in the process, will using FreeDOS allow my program to access more than the standard 640K of memory?

And secondly, about the ASM, I know on modern processors it is hard to program on assembly due to the complexity of the CPU architecture, but does using FreeDOS limit me to the presumably simpler 16-bit instruction set?

Answer 1

Without any extender a program can use maximum 640KB of low memory in DOS. But each structure will be limited to the size of a segment, or 64KB. That means you can have 10 large arrays of size 64KB. Of course you can have multiple arrays in a segment but their total size must not exceed the segment size. Some compilers also handle addresses spanning across multiple segments automatically so you can use objects larger than 64KB seamlessly, or you can also do the same if you're writing in assembly

To access more memory you need an extender like EMS or XMS. But note that the address space is still 20-bit wide. The extenders just map the high memory regions into some segments in the addressable space so you can only see a small window of your data at a time

Regarding assembly, you can use 32-bit registers in 16-bit mode. There are 66h and 67h prefixes to change the operand size. However that doesn't mean that writing 16-bit code is easier. In fact it has lots of idiosyncrasies to remember like the limited register usage in memory addressing. The 32-bit x86 instruction set is a lot cleaner with saner addressing modes as well as a flat address space which is a lot easier to use.

Answer 2

MS-DOS and FreeDOS use the "HIMEM" areas: These are:

• Some memory areas above 0xA000:0x0000 reserved for extension cards that contain RAM instead of extension cards
• The memory starting from 0xFFFF:0x0010 to 0xFFFF:0xFFFF which is located above 1MB but can be accessed using 16-bit real mode code (if the so-called A20-line is active).

The maximum memory size that can be archieved this way is about 800K.

Using XMS and EMS you can use up to 64M:

• XMS will allocate memory blocks above the area that can be accessed via 16-bit real mode code. There are special functions that can copy data from that memory to the low 640K of memory and vice versa
• EMS is similar; however using EMS it is possible to "map" the high memory to a low address (a feature of 32-bit CPUs) which means that you can access some memory above the 1MB area as if it was located at an address below 1MB.