You could use the example source given in the SDCC manual ( http://sdcc.sourceforge.net/doc/sdccman.pdf ) chapter 3.5 "Absolute Addressing" as a starting point.

It compiles to the code below. For Keil which I assume you'll be using you'd have to use r7 instead of dpl for parameter passing.

Then strip off a few byte from the SDCC generated code below:

;------------------------------------------------------------
;Allocation info for local variables in function 'spi_io'
;------------------------------------------------------------
;out_byte                  Allocated to registers r2
;i                         Allocated to registers r3
;------------------------------------------------------------
;spi_io.c:8: unsigned char spi_io(unsigned char out_byte)
;       -----------------------------------------
;        function spi_io
;       -----------------------------------------
_spi_io:
        mov     r2,dpl; <------------------ adapt

;spi_io.c:11: do {
        mov     r3,#0x08
00103$:
;spi_io.c:12: MOSI = out_byte & 0x80;
;spi_io.c:13: out_byte <<= 1;
        mov     a,r2
        add     a,r2
        mov     _MOSI,c
        mov     r2,a
;spi_io.c:14: MCLK = 1;
        setb    _MCLK
;spi_io.c:16: if(MISO)
        jnb     _MISO,00102$
;spi_io.c:17: out_byte += 1;
        inc     r2
00102$:
;spi_io.c:18: MCLK = 0;
        clr     _MCLK
;spi_io.c:19: } while(--i);
        djnz    r3,00103$
00114$:
;spi_io.c:20: return out_byte;
        mov     dpl,r2 ; <----------------- adapt
        ret                               

Greetings,
Frieder