1. LHO 15 C with assembly language
EGE 631
LHO 15
C with assembly language

2. There are several reasons to call an assembly routine from your C program.
You may have assembly code already written that you wish to use
You may need to improve the speed of a particular function
You may want to manipulate SFRs or memory-mapped I/O devices directly from assembly
Segment Naming Conventions

3. Data Objects - Data objects are the variables and constants you declare in your C programs.
The Cx51 compiler generates a separate segment for each memory type for which a variable is declared. The following table lists the segment names generated for different variable data objects.

6. Parameter Passing in Registers - C functions may pass parameters in registers and fixed memory locations. A maximum of 3 parameters may be passed in registers. All other parameters are
passed using fixed memory locations. The following tables define what registers are used for passing parameters.

7. The following examples clarify how registers are selected
for parameter passing.

8. Function Return Values - Function return values are always passed using CPU registers. The following table lists the possible return types and the registers used for each.

11. ; ASM1.SRC generated from: ASM1.C
NAME ASM1
?PR?_asmfunc1?ASM1 SEGMENT CODE
PUBLIC _asmfunc1
; #pragma SRC
; #pragma SMALL
; unsigned int asmfunc1 (
RSEG ?PR?_asmfunc1?ASM1
USING 0
_asmfunc1:
;---- Variable 'arg?00' assigned to Register 'R6/R7' ----
; SOURCE LINE # 4
; SOURCE LINE # 6
; return (1 + arg);
; SOURCE LINE # 7
MOV A,R7
ADD A,#01H
MOV R7,A
CLR A
ADDC A,R6
MOV R6,A
; }
; SOURCE LINE # 8
?C0001:
RET
; END OF _asmfunc1
END

12. $include (c8051f020.inc); Register definition file. NAME MBTN
?PR?mbtn?MBTN SEGMENT CODE
?DT?mbtn?MBTN SEGMENT DATA OVERLAYABLE
EXTRN CODE (rd_buttons)
PUBLIC mbtn
RSEG ?DT?mbtn?MBTN
?mbtn?BYTE:
last_button: DS 1
RSEG ?PR?mbtn?MBTN
;***************************************************************
; FUNCTION: mbtn - C callable routine REMEMBERS WHICH BUTTON WAS
; PRESSED LAST. IF NO BUTTONS HAVE BEEN PRESSED
; ASSUMES P5.3 BUTTON WAS LAST ONE PRESSED.
; INPUTS: NONE
; OUTPUTS: R7(3..0) - BITS ARE SET BASED ON LAST BUTTON PRESSED
; NOTE: ONE AND ONLY ONE BIT OF ACC(3..0) IS SET.
; R7(7..0) = UNDEFINED
; CALLS: rd_buttons
; DESTROYS: ACC, PSW, R7
; Bytes of code

13. mbtn:
USING 0
CALL rd_buttons ;GET BUTTONS
MOV A,R7
JZ FIX_BTNS ;JUMP IS NO BUTTONS PRESSED
MOV last_button,A ;SAVE BUTTONS
;BE SURE ONE AND ONLY ONE BUTTON SAVED
FIX_BTNS:
MOV A,#01 ;SET BUTTON PRESSED BIT
XCH A,last_button ;SWAP LOW NIBBLES
SETB ACC.3 ;BE SURE AT LEAST ONE BIT IS SET
CHKBTN: RRC A ;CHECK FOR A 1
JC GOT1 ;EXIT IF 1
XCH A,last_button ;GET LSTBTN AND
RL A ;SHIFT BIT IN LSTBTN
XCH A,last_button ;SAVE LSTBTN
JMP CHKBTN ;KEEP LOOKING FOR FIRST 1
GOT1: MOV R7,last_button ;RETURN LAST BUTTON PRESSED
RET
END