INMC 80 News


October–December 1981, Issue 5

Page 47 of 71

Line 4515 tells us we are about to output the low nibble of A.

Line 4520. Now the fun really starts. The rotate instructions have left a lot of indeterminate garbage in the high nibble. Indeterminate, because we niether knew nor cared what the state of the C flag was before the start of the rotates. We want to get rid of the high nibble, so we use a logical AND instruction. We AND the X5H with 0FH, which like magic gives us 05H. Ok, so how is it done? Well I don’t intend to go into a discourse on Boolean algebra here, save to say that in the middle of the last century a fella called Boole (french, I think), proposed a ‘logical’ method of performing algebraic functions which caused a few sniggers at the time. He suggested that there shouldn’t only be the normal arithmetic operators like, plus, minus, multiply and divide, but there should also be what he call logical operators like AND, NOT and OR, etc. No-one payed much attention to him and what he had to say didn’t really become relevant until way after his death. I often wonder what would have happened if Boole and Babbage had got together, may be we would have had vast steam driven mechanical computing engines to rival Asimov’s Multivac, and this in the middle of the last century. The nett result of ANDing a logical 1 with a logical 1 is 1 (and no carry), and of ANDing logical X (either 1 or 0) with 0 is 0.


See!! If you don’t, then get Boole’s book out of the library, it’s still in print, and his methods will get more important as you progress in this business. Clever, what? We’ve got rid of the bits we didn’t want, and are left with those we want. The process has a name, it’s called masking. The 0FH is the mask, and it allows through what is required, whilst holding back that which is note

Line 4525. We’ve now got to convert the four bits in a into an ASCII character which will represent the four bits in HEX. And that’s not so easy. We start off by filling the high nibble with the binary pattern for 9, 1001, and that is achieved by ADDing 90H to A. The ADD instruction is used, as the C flag is still indeterminate, and we don’t want any spurious carries added to our low nibble. We have used the ADD A,90H instruction, because we are going to use a Decimal Add Adjust (DAA) instruction next. By having 9 as the high nibble, it is our intention to make the high nibble overflow if there is an overflow from the low nibble. The ADD instruction also set the ‘Half Carry’ flag as the A register contents are now 9BH, the overflow being to warn that the low nibble contains a number greater than (decimal) 9, in case there is a following DAA instruction. Please think carefully, I’m doing my best.

Line 4530. We have 9BH in the A register, and we perform a Decimal Add Adjust on it. Now the low nibble was B, which the DAA instruction takes as 11 (decimal), so it puts 1 in the low nibble. As the low nibble was 11, the previous ADD instruction had already set the ‘Half Carry’ flag, so there is a carry through to the high nibble, making it 10 (decimal), so it puts 0 in the high nibble, and sets the Carry flag to show there was an overflow. The A register now contains 01H, and the C flag has been set to indicate an overflow.

Line 4535. We’re half way there, all that’s needed now is to add the ASCII offset to the number in the A register, take account of any carries, and we’re all but home. Now, the ASCII code for 0 (nought) is 30H, 1 is 31H, upto 39H for 9. Fortunately for our purposes, the ASCII code for ‘A’ is 41H, though to 46H for ‘F’. Let’s add (with carry) 40H to the A register.

01000010which is 42H

Page 47 of 71