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Example of 80X86 Assembler Code
The following example shows the typical structure of the software that
I write, and the level of commenting present:
;******************************************************
;*** INTERFACE ROUTINES ***
;******************************************************
;
Read_Char PROC ;read char from input buffer
;
;Entry: CL = port number
;
;Exit: AL = char (z set if none)
;
push cx ;preserve registers
push bx
;
cmp cl,0 ;only port 0 allowed
jz rc_1
;
_Int_Error 03301h ;invalid port number
;
rc_1: mov al,Rx_InPtr
sub al,Rx_OutPtr ;inptr - outptr = num. chars
jz rc_zex ;if zero exit
;
push ax ;save inptr - outptr
;
mov bl,Rx_OutPtr
mov bh,0
add bx,OFFSET Rx_Buf ;point to char
mov al,[bx] ;get char
inc Rx_OutPtr ;inc outptr
;
mov ah,Control
shl ah,1
shl ah,1 ;7/8 bits now in bit 7
and ah,10000000b ;mask lower bits
or ah,01111111b ;set to 1's
and al,ah ;combine with char
mov ch,al ;temp save char
;
pop ax ;recover inptr - outptr
dec al ;adjust
cmp al,MARGIN_LOW
jc rc_hsk ;hsk needs to be turned on
;
rc_ex: or al,0FFh ;reset z
mov al,ch ;get char back
pop bx ;recover registers
pop cx
;
ret
;
;
rc_zex: xor al,al ;set z
pop bx ;recover registers
pop cx
;
ret
;
;
rc_hsk: ;turn data back on
;don't use ch (char)
mov al,In_Hsk ;get hsk
and al,al
jz rc_ex ;hsk already 'go' - exit
;
mov In_Hsk,GO ;set to 'go'
test Control,00010000b ;parallel / serial ?
jnz rc_par
;
rc_ser: test Control,00000100b
jz rc_rts ;non xon hsk
;
test Control,00000010b
jz rc_xdup ;xon, duplex
;
;xon, input
mov al,XON
mov dx,COM1_DATA
out dx,al ;send xon
jmp SHORT rc_ex ;exit
;
;
rc_xdup: ;xon, duplex
cli ;must not have ptrs change
mov dx,COM1_LSR
in al,dx
and al,00100000b ;Tx holding reg empty
jz rc_xdup1
mov al,XON ;empty
mov dx,COM1_DATA
out dx,al ;send xon
sti ;int ok
jmp SHORT rc_ex ;exit
;
rc_xdup1:
dec Tx_OutPtr ;insert char at end of buffer
mov bl,Tx_OutPtr
mov bh,0
add bx,OFFSET Tx_Buf
mov BYTE PTR [bx],XON ;xon at end of buffer
sti ;int ok
jmp SHORT rc_ex ;exit
;
;
rc_rts: mov dx,COM1_MCR
in al,dx ;read MCR current
or al,00000011b ;set RTS, DTR on
out dx,al
jmp SHORT rc_ex ;exit
;
;
rc_par: _Int_Error 03302h ;parallel not supported
;
Read_Char ENDP
;
;
;
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