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From: hollen@zeta.megatek.uucp (Dion Hollenbeck)
Newsgroups: alt.msdos.programmer,comp.sys.ibm.pc
Subject: Re: Calling C functions from assembly
Message-ID: <663@megatek.UUCP>
Date: 2 Aug 89 14:11:42 GMT
References: <12324@s.ms.uky.edu>
Sender: news@megatek.UUCP
Lines: 84

From article <12324@s.ms.uky.edu>, by simon@ms.uky.edu (G. Simon Gales):
> I'm trying to call some C functions from assembly, using MSC 5.0 and either
> MASM or TASM.  I could write a dummy _main() in C, if this makes things
> easier, but I'm still not sure what needs to be setup to call a C subroutine.
> 
> Can anyone clue me in on how to do this?

There is a section in the ref. manual dealing specifically with this,
but I will summarize.  Be aware that I am summarizing from memory
and do not have the manual in front of me.  I will address both calling
C functions from assembly and assembly being called by C.

Calling C from Assembly.
=======================
Pseudocode the calling interface so you know the order of the arguements.
Push the arguements onto the stack from right to left and push
multi-word args (floats, far ptrs....) from high to low.  Be sure of
your memory model so that you can have the segment registers OK.  Some
memory models ASSUME ES = DS so you must make it so before calling C
functions.  For instance the tiny memory model assumes that 
CS = DS = ES = SS.  Results are returned in registers.  A char in AL, a
short int in AX, a long int or float in AX/BX (low order in BX),
and a double in AX, BX, CX, DX high order in AX to low order in DX.


Calling Assembly from C.
========================
Here is a code fragment for ASM being called by C.

;;  FUNCTION
;;	read_ee_raw (ee_offset, numbytes, destination)
;;
;;	int	ee_offset		ARG1
;;	int	num_bytes		ARG2
;;	char far	*destination	ARG3 - offset
;;					ARG4 - segment

	PUBLIC	READ_EE_RAW

READ_EE_RAW	PROC	NEAR

	PUSH	BP			;set up frame pointer
	MOV	BP,SP

	PUSH	AX			;save registers used
	PUSH	DI
	PUSH	SI
	PUSH	DS
	PUSH	ES

	LDS	SI,[EEPROM_BASE]	;get seg:off ptr to EEPROM base
	MOV	AX,[BP+ARG1]		;get byte offset into EEPROM
	SHL	AX,1			;make word offset
	ADD	SI,AX			;add to base offset
	MOV	DI,[BP+ARG3]		;get offset to destination
	MOV	AX,[BP+ARG4]		;get segment of destination
	MOV	ES,AX			;get in dest segreg
	MOV	CX,[BP+ARG2]		;get count of bytes to read

EEPROM_READ_LOOP:
	LODSW				;read word - high byte is junk
	STOSB				;write byte
	LOOP	EEPROM_READ_LOOP	;continue until done


	POP	ES			;restore registers used
	POP	DS
	POP	SI
	POP	DI
	POP	AX

	POP	BP			;restore frame pointer

	RET
READ_EE_RAW	ENDP


No other explanation should be necessary except to say to read the
manual to be sure what registers should be saved and restored.  You
cannot go wrong by saving/restoring all you use. 
	Dion Hollenbeck             (619) 455-5590 x2814
	Megatek Corporation, 9645 Scranton Road, San Diego, CA  92121

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