ExecAns shouldn't create a program, it runs a program. It takes the code in Ans, puts it into prgmU, and runs prgmU. The problem might be that the current running program (the one that calls prgmEXECANS) gets overwritten when prgmU gets run. I'm not sure, i'll look at it later.

But this isn't really about writing assembly in hex anymore, you might want to open a new thread asking for help on this specific program. But it would really help if you provided the mnemonics instead of the hex so people other than Xeda and perhaps calc84maniac actually knew what the code did. Hex is such a pain.
;(saferam1 = saveSScreen)
name_str = saferam1
size = saferam1+2
code_start = saferam1+4

start:
    bcall(_RclAns)            ;set pointer into OP1
    cp 4
     ret nz
    ld hl,$55                ;ASCII for 'U'
    ld (name_str),hl        ;save zero terminated string name
    ld (OP1+1),hl
    ld hl,code_start
    ex de,hl                ;de = code_start, hl = ans ptr?
    ld c,(hl)                ;i assume bc = size of Ans
    inc hl
    ld b,(hl)
    inc hl
    ld (size),bc
    ldir                    ;copy code in Ans into saferam
    bcall(_ChkFindSym)
     jr c,not_found
        bcall(_DelVarArc)    ;del prgmU if it existed
not_found:
    ld a,5                    ;5 = non-protected program
    ld hl,saferam1
    bcall(_ExecuteNewPrgm)    ;a = var type, hl = pointer to structure (zero terminated string, size (word), program data)
    retA couple things: i'm not sure if RclAns actually gives readable results in a/de/hl. They seem to assume that _RclAns puts out the same outputs as _ChkFindSym (see WikiTI). WikiTI and the 83 Plus System Routines manual both say all registers are destroyed (ie. return junk/non-useful info). I don't know what happens with _ExecNewPrgm, it may kick the currently running BASIC program out.

EDIT: If all you want to do is create a program, you should look at the _CreateProg bcall.

That's the same code as from TI-BasicDev just in assembly mnemonics. I'm not really sure what you want, when you run that program does it return to the TI-OS? Like i said there, there's another routine to call if all you want to do is create a program (_CreateProg).

Sorry, I am bad at explaining... I know HEX and ASM are the same things, the first one is just ASM translated to the calculator's language.
Or am I wrong?!

So I am trying again to learn ASM in 28 days (it's a lot easier the second time) but TASM does not work on my new 64 bit notebook. I downloaded SPASM but now even the copy-pasted hello world program makes it throw errors on each b_call and I really don't know what went wrong... I suppose the TASM syntax is a bit different from the SPASM syntax?

Ok, thank you, I'll try this when I'll have more time... (If I ever will have more time... )

What doesn't work? Assembling it? Are you using the ti83plus.inc that i linked to? What exactly are you trying to assemble (can you post the code)? Does it not assemble or does the .8xp not work?

The underscores need to be in there afaik.

I was talking about the underscore in the B_CALL macro