C51 Compiler - Advanced Programming Techniques (3) - Interface between C language and assembly

Cx51 programs can be easily interfaced with 8051 assemblers. The A51 assembler is an 8051 macro assembler that emits object modules in OMF-51 format. By observing some programming rules, you can call assembler programs from C programs and vice versa. Public variables declared in assembly modules can also be used in C programs.

There are several reasons to call an assembler from a C program. First, you can use an existing assembler, second, you want it to run faster, and third, you want to use it directly in assembly to manipulate SFRs or use I/O memory images.

For an assembler to be called from a C program, its parameter passing rules and value return rules must be consistent with C functions. In terms of application, it must look like a C function.

Function Parameters

In general, a C function can pass three parameters through registers. The other parameters are passed through fixed memory. Of course, NOREGPARAMS can be used to prohibit the use of registers to pass parameters. If it is specified that registers are not allowed to pass parameters or there are too many parameters, fixed memory areas are used to pass parameters. When generating code, the function that passes parameters in registers has "_" added to the front of the name, and the function that uses fixed memory areas to pass parameters does not change its name.

Parameter Passing in Registers

Passing parameters in registers

C functions can pass parameters in registers or in fixed storage areas. Up to three parameters can be passed in registers. The following table shows the rules for parameter passing:

Arg Number char, 1-byte ptr    int, 2-byte ptr     long, float     generic ptr

1 R7 R6 & R7 R4—R7 R1—R3

(MSB in R6,         (Mem type in R3,

LSB in R7)              MSB in R2

LSB in R1)

2          R5              R4 & R5     R4—R7         R1—R3

(MSB in R4,             (Mem type in R3

LSB in R5)                  MSB in R2

                                                    LSB in R1)

3          R3              R2 & R3                     R1—R3

(MSB in R2,                 (Mem type in R3

LSB in R3)              MSB in R2,

LSB in R1)

Example

Declaration       Description

func1 (int a) parameter a is passed through R6 and R7

func2 (int b,int c,int *d) parameter b is passed through R6 and R7, parameter c is passed through R4 and R5, and parameter d is passed through R1, R2 and R3

func3 (long e, long f) parameter e is passed through R4, R5, R6, R7, and parameter f is passed through a fixed storage area

func4 (float g, char h) parameter g is passed through R4, R5, R6, R7, and parameter h is passed through a fixed storage area

Parameter Passing in Fixed Memory Locations

Passing parameters via fixed storage area

Procedures that pass parameters through fixed storage areas ?function_name?BYTE and ?function_name?BIT name and carry parameter values ​​and pass them to function_name. Bit parameters are copied to the ?function_name?BIT segment before calling the function. All other parameter data is copied to the ?function_name?BYTE segment. All parameters are assigned storage space within these segments, even if they may use registers to pass parameters. Parameters are stored in the corresponding segments in the order they are declared.

Depending on the storage mode, the fixed storage area for parameter transfer may be the internal data memory or the external data memory. The small storage mode uses the internal data memory as the parameter transfer area, which is the most efficient. The Compact and large storage modes use the external data memory as the parameter transfer area.

Function Return Values

The return value of a function always uses CPU registers. The following table lists the registers used by possible return values.

Return Value Type Register Description

bit Carry Flag A single bit is returned in the carry position

char / unsigned char, R7 A single byte is returned in R7

1-byte pointer

int / unsigned int, R6 & R7 high bit in R6, low bit in R7

2-byte ptr

long / unsigned long R4-R7 The high bit is in R4, the low bit is in R7

float R4-R7 32-bit IEEE format

generic pointer R1-R3 memory type in R3, high in R2, low in R1

Using the SRC Directive

You can use the Cx51 compiler to generate an assembly code program, and use this program to determine the transfer rules that should be used. The instruction SRC can specify that Cx51 generate an assembly program instead of an object program. For example, the following C program:

#pragma SRC

#pragma SMALL

unsigned int asmfunc1 (

unsigned int arg)

{

return (1 + arg);

}

Generates the following assembler:

; 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:

RIGHT

; END OF _asmfunc1

END

In this example, the function name asmfunc1 is prefixed with an underscore in the assembler to indicate that the parameters are passed through registers. The parameter arg is passed through registers R6 and R7.

The following program is an assembler compiled from the same source program, except that the NOREGPARMS directive is used:

; ASM2.SRC generated from: ASM2.C

NAME    ASM2

?PR?asmfunc1?ASM2   SEGMENT CODE

?DT?asmfunc1?ASM2   SEGMENT DATA

PUBLIC ?asmfunc1?BYTE

PUBLIC asmfunc1

RSEG ?DT?asmfunc1?ASM2

?asmfunc1?BYTE:

arg?00: DS 2

; #pragma SRC

; #pragma SMALL

; #pragma NOREGPARMS

;

; unsigned int asmfunc1 (

RSEG ?PR?asmfunc1?ASM2

USING 0

asmfunc1:

; SOURCE LINE # 5

; SOURCE LINE # 7

; return (1 + arg);

; SOURCE LINE # 8

MOV A,arg?00+01H

ADD A,#01H

MOV R7,A

CLR A

ADDC A,arg?00

MOV R6,A

; }

; SOURCE LINE # 9

?C0001:

RIGHT

; END OF asmfunc1

END

 Note that in the example, there is no underscore prefix to the function name asmfunc1, and the parameters are passed in the ?asmfunc1?BYTE segment.