0001   0000             ;Z-80 Highest Factor Program
0002   0000             ;Donn Stewart
0003   0000             ;2/22/03
0004   0000             
0005   0800             	.org     0800h		;Start of RAM--load program here.
0006   0800 DB 00       strt    in      a,(0)		;Get number to factor from input port 0
0007   0802 32 21 08    	ld      (original_number),a	;Store it
0008   0805 32 22 08    	ld      (test_factor),a	;Use it as the initial test_factor
0009   0808 3A 22 08    loop1   ld      a,(test_factor)	;Main loop of program
0010   080B 3D          	dec     a		;Make new test_factor by decrementing
0011   080C 28 0E       	jr	z,end		;If zero, quit (all done)
0012   080E 32 22 08    	ld      (test_factor),a	;If not, store and test
0013   0811 47          	ld      b,a		;Put test_factor in reg b
0014   0812 3A 21 08    	ld      a,(original_number);Put original_number in reg a
0015   0815 90          loop2   sub     b		;Test by subtracting repeatedly
0016   0816 28 04       	jr      z,end		;Exact factor--zero result		
0017   0818 30 FB       	jr      nc,loop2	;Result non-negative, keep trying
0018   081A 18 EC       	jr      loop1		;Result negative, not a factor
0019   081C 78          end     ld      a,b		;Display factor, or zero if prime
0020   081D D3 00       	out     (0),a
0021   081F 18 DF       	jr      strt		;Start over
0022   0821 00          original_number .byte       00h
0023   0822 00          test_factor     .byte       00h
0024   0823             	.end
0025   0823             
0026   0823             
tasm: Number of errors = 0