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?
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.