void ext_int_0(void) interrupt 0 	;external interrupt 0
{                       
#pragma asm                     ;SRC directive  must be active to use this.
    clr EA                      ;disable interrupts
        
    jb  PS2_DATA, the_end       ;start bit must be 0
        
    mov R0, #08                 ;bits in a byte counter
    clr A

getbyte:

; look for +ve edge
wait_till_clk_high:             ;==> while(clock == low);
    jnb PS2_CLK, wait_till_clk_high

; look for -ve edge             
wait_till_clk_low:              ;==> while(clock == high);
    jb PS2_CLK, wait_till_clk_low
; little delay
    mov R1, #8
    djnz r1, $                  ;reading LSB 20us after -ve edge
; okay go on...
    mov c, PS2_DATA
    rrc a                       ;LSB is clokced in first
    djnz  R0,getbyte            ;repeat for whole byte  
/*----now byte is in ACC----*/  ;we have only 900 us to waste

finally:
    mov R0, write_counter       ;i.e. #0x00
    movx @R0, A                 ;save ACC to external RAM (8-bit addr)
    inc write_counter

    mov A, write_counter        
    xrl A, #200                 ;write_counter: 0-199 are valid 
    jz  pwned                   ;clear write_counter if = 200
    ajmp wait                   ;else exit ISR 

pwned:
    mov write_counter,#0        ;clear write_counter

wait:
    mov R1, #200                 ;waste ~400us to allow the parity & stop bits to pass...
    djnz R1, $                   ;MUST WASTE MIN 224us
    mov R1, #200         
    djnz R1, $          

the_end:
    setb EA
#pragma endasm
/*===========================================================*/ ;end of inline ASM
};ISR ends