Why doesn't MS-DOS initialize the DS and ES registers?
assembly, dos, intel, masm, x86-16
Solution
Because `CS` and `SS` registers are essential for program execution in contrast to `DS` and `ES` registers which point to user-defined data segments. By default no data is present in the executing program this nothing to initialize the `DS` and `ES` with. As a program writer you can specify where your data is by setting the data segments registers.
Edit: as was correctly noted by @FrankKotler, in `.com` file (the entire program size doesn't exceed single segment), `DS` and `ES` are initialized and equal to `CS`. For other execution models, `DS` and `ES` are initialized by pointer to `PSP` (which isn't the pointer to real user data).
Problem
Why does the initialization of the `DS` and `ES` registers has to be done manually by the programmer? For example: ``` MOV AX, DTSEG MOV DS, AX ``` On the other hand, the `CS` and `SS` registers are initialized by the operating system (in `MS-DOS`). Why is this so?