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> DOS > Working with FPU insturctions |
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AsmGuru62 02 Mar 2013, 13:42
You can pass two parameters:
1. Address of location, where the result is stored 2. Address of location (or locations), where the input is stored So, in your function you deal with addresses as parameters. I tried a small COM project for this: MAIN.ASM: Code: ; --------------------------------------------------------------------------- ; FILE: DosComTemplate.Asm ; DATE: March 2, 2013 ; --------------------------------------------------------------------------- org 100h use16 ; {INSMODDEF} Module Definitions will be inserted here ; ; MS-DOS Program Entry Point ; ; ; Parameters are pushed in opposite order ; (you probably need a macro to make it shorter) ; mov ax, divisor push ax mov ax, m2 push ax mov ax, m1 push ax mov ax, result push ax call Module1_PerformMulDiv ; (result, m1, m2, divisor) ; ; Just to check result in debugger ; fld [result] fstp st0 ; Throw it out ; ; Quit to MS-DOS ; int 20h include 'Module1.Asm' ; {INSMODIMPL} Module Implementations will be inserted here ; ; Some DOUBLEs... ; align 8 m1 dq 43.9548 m2 dq 79064.03262 divisor dq 6534.77544 result dq 0.0 MODULE1.ASM: Code: ; --------------------------------------------------------------------------- ; FILE: Module1.Asm ; DATE: March 2, 2013 ; --------------------------------------------------------------------------- align 32 Module1_PerformMulDiv: ; --------------------------------------------------------------------------- ; INPUT (stdcall addresses of DOUBLE values): ; RESULT, MUL1, MUL2, DIVISOR ; OUTPUT: ; Expression (MUL1*MUL2)/DIVISOR will be stored at address in RESULT ; --------------------------------------------------------------------------- push bp mov bp, sp push bx ; ; BP+4 -> RESULT ; BP+6 -> MUL1 ; BP+8 -> MUL2 ; BP+10 -> DIVISOR ; mov bx, [bp + 6] fld qword [bx] mov bx, [bp + 8] fmul qword [bx] mov bx, [bp + 10] fdiv qword [bx] mov bx, [bp + 4] fstp qword [bx] pop bx bp ret 4*2 ; Restore stack for 4 16-bit parameters For an EXE file and FAR pointers you will have to do some adjustments. I must add that the convention for a function returning a single float point value is to return it in ST0 register of the FPU. So, you can use it instead of storing it at some address. Same can be said for an input parameters to a function --- they also can be passed in registers. |
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02 Mar 2013, 13:42 |
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SeproMan 02 Mar 2013, 13:48
wyvern,
The dword at [bp+2] is not what you need! You should move the word at [bp+4] in an address register (BX, SI, or DI). Note that if you preserve this register also, you must use [bp+6] instead! Why not return the result at the same place you found the input? _________________ Real Address Mode. |
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02 Mar 2013, 13:48 |
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wyvern 02 Mar 2013, 19:30
AsmGuru62, SeproMan:
Thank very much you for your help, now i realize i was close to the solution (sorry for the "[bp+2]", i mean [bp+4]). And about "calling conventions", if return value must be in ST0, what happen if i want to return two or more values. For example, suppose i have a function that evaluates a 2ยบ polinomyal and i want to return two roots... i could use st0 and st1?, or would be better to receive two memory buffers from the caller to store the two results there?. I just want to know wich is the most "correct" or convenient way to do it. Thanks |
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02 Mar 2013, 19:30 |
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AsmGuru62 03 Mar 2013, 19:07
I would use ST0 and ST1.
The calling conventions are for interfacing your function with HLL. Lets say that your function will be called by C/C++ -- in this case you need to follow the convention. Otherwise, if function used only in your own ASM code, then you can do whatever produces smaller code -- I think ST0,ST1 combination will produce shorter code. |
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03 Mar 2013, 19:07 |
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wyvern 03 Mar 2013, 19:12
AsmGuru62, im more interested to write code that can be easily used from anywhere (or at least not just from my own code). In this case, what would be the better aproach?
Thanks. |
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03 Mar 2013, 19:12 |
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AsmGuru62 04 Mar 2013, 18:01
This article is not bad at the subject:
http://en.wikibooks.org/wiki/X86_Disassembly/Floating_Point_Numbers |
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04 Mar 2013, 18:01 |
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wyvern 04 Mar 2013, 22:40
Excelent info, thanks for the link.
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04 Mar 2013, 22:40 |
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ACP 04 Mar 2013, 23:39
wyvern wrote: Im learning to work with the FPU on a 16 bit enviroment, im using DOSBOX and currently the TASM assembler because i have a lot of source examples made in TASM, but mi intentions are to continue with FASM. Please note that TASM (I'm assuming you are using 5.x) is adding WAIT/FWAIT instruction before any FPU instruction unless you specify CPU type to be 486 or higher (if I remember correctly). If 486 CPU has been selected TASM skips it just like FASM does. So if you "port" code generated by TASM to FASM you have to remember about it. Just my 2 cents... |
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04 Mar 2013, 23:39 |
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