r0 = 0 r1 = 1 r2 = 2 r3 = 3 r4 = 4 r5 = 5 r6 = 6 r7 = 7 r8 = 8 r9 = 9 ra = 10 rb = 11 rc = 12 rd = 13 re = 14 rf = 15 #define Low(x) ((x) & 255) #define High(x) ((x) / 256)