; this is an example of out instruction. 
; it writes values to virtual i/o port 
; that controls the stepper-motor. 
; c:\emu8086\devices\stepper_motor.exe is on port 7 

#start=stepper_motor.exe#


name "stepper"

#make_bin#

steps_before_direction_change = 20h ; 32 (decimal) 

jmp start

; ========= data =============== 

; bin data for clock-wise 
; half-step rotation: 
datcw    db 0000_0110b
         db 0000_0100b
         db 0000_0011b
         db 0000_0010b

; bin data for counter-clock-wise 
; half-step rotation: 
datccw   db 0000_0011b
         db 0000_0001b
         db 0000_0110b
         db 0000_0010b


; bin data for clock-wise 
; full-step rotation: 
datcw_fs db 0000_0001b
         db 0000_0011b
         db 0000_0110b
         db 0000_0000b

; bin data for counter-clock-wise 
; full-step rotation: 
datccw_fs db 0000_0100b
          db 0000_0110b
          db 0000_0011b
          db 0000_0000b


start:
mov bx, offset datcw ; start from clock-wise half-step. 
mov si, 0
mov cx, 0 ; step counter 

next_step:
; motor sets top bit when it's ready to accept new command 
wait:   in al, 7
        test al, 10000000b
        jz wait

mov al, [bx][si]
out 7, al

inc si

cmp si, 4
jb next_step
mov si, 0

inc cx
cmp cx, steps_before_direction_change
jb  next_step

mov cx, 0
add bx, 4 ; next bin data 

cmp bx, offset datccw_fs
jbe next_step

mov bx, offset datcw ; return to clock-wise half-step. 

jmp next_step