fcom | fast file commander | File Utils library

The warning message tells you exactly which line is causing the problem, and why.

np.hstack does not (or won't) accept generator expressions as an input. The following line contains a generator expression:

Only the 8087 requires a WAIT prefix before each floating point instruction (unless you manually count out the cycles to ensure that enough time has passed for the operation to have finished). Starting with the 80286 and the 80287, the main processor would wait for coprocessor operation to finish before issuing any FPU instructions on its own without having to rely on a WAIT instruction. An explicit WAIT prefix is only needed if you want to observe a store or exception performed by the FPU.

For this reason, assemblers generally leave out WAIT prefixes when the CPU selected is an 80286 or later. The instructions for which a WAIT prefix is generated are instructions that could discard or affect pending floating point exceptions. A WAIT prefix is generated to make sure that any floating point exception is delivered before the instruction executes. Separate variants of these instructions prefixes FN instead of F are usually available if this is not desired.

An 8087 coprocessor cannot be used with an 80286 or later main processor, so I suppose MASM treats the .8087 directive as “coprocessor present” rather than “8087 present.” Thus, only the main processor selected distinguishes whether WAIT prefixes are emitted.

I was dumb and didn’t know the difference between execution: creating and running it

(Just expanding on my comment, so that this gets an answer.)

For background: the complicated sequence of instructions for floating-point compares comes from the fact that early x86 CPUs didn't have the FPU on-board; it was an optional separate chip, and its ability to interact with the CPU was limited. So the FCOM instruction couldn't set the CPU's FLAGS register directly. Instead, it sets the floating point status word, which was internal to the floating-point coprocessor. The FSTSW instruction could be used to get the status word from the coprocessor and load it into a general-purpose CPU register, and then SAHF would get the appropriate bits of AH and write them to FLAGS.

After all this, you finally get the FLAGS set to indicate the result of the comparison, and the bits of the status word are laid out so as to set FLAGS the same way as for an integer comparison: ZF will be set if the numbers were equal, CF if the difference was strictly negative, and so on. So you can now use conditional jumps like ja, jb, etc, just as you would for unsigned integer comparisons. Note that PF=1 implies the comparison was unordered (at least one operand was NaN), so you need to check that first.

(PPro added FCOMI which sets EFLAGS from the FP compare the same way fcom/fstsw/sahf does, avoiding the extra instructions. See also Why do x86 FP compares set CF like unsigned integers, instead of using signed conditions?)

However, your code has add [i], 1 in between, and like most x86 arithmetic instructions, it sets FLAGS based on the result. So your carefully retrieved FLAGS are overwritten, and the jb a couple lines down is based on the result of the add instead of the FCOM. Thus you need to rearrange those.

For example, do add before SAHF. Or before fcomi.

The lecture_room variable is of type int. The strcpy(...) function expects char buffer as destination. You have provided int instead, so strcpy(...) will write strings into location with memory address equal to the lecture_room value. Thus, into random memory location, leading to random effects (e.g. stackoverflow, values overwrite and so on).