The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Calling Assembly-language Routines
Listing 13-5. Assembly-language sine-wave routine for Figure 13-2’s circuit..
;Reads and copies values in sequence from locations 3000h
;to 30FFh to E400h. A DAC0832 generates a sine wave from
;the values.
;A keypress terminates the routine and returns to BASIC-52.
org 3100h ;load routine above the
;stored values
OutputPort equ 0e400h ;address of port to write
;sine values to
;Begin generating the sine wave:
nextcycle mov 20h,#0ffh ;store initial count at 20h
;in internal
;data memory
nextvalue mov dph,#30h ;put high byte of sine wave
;address (30h)
;in dptr
mov dpl,20h ;copy low byte of sine wave
;address from 20h to dptr
movx a,@dptr ;place sine wave value in
;acc.
mov dptr,#OutputPort ;copy port address
;to dptr
movx @dptr,a ;copy sine wave value to
;output port
djnz 20h,nextvalue ;do another if count > 0
jb ri,return ;after writing one complete
;cycle, check serial receive
;flag and quit if set
sjmp nextcycle ;if serial flag not set,
;begin another cycle
;Return to BASIC-52:
return clr ri ;clear serial receive flag
ret
;return to BASIC
end
The Microcontroller Idea Book 229