Beginning ML Programming Continued

 Continuing from the last article by The Wild One , we will now go into the use of the three different registers in which he covered . . There are several more in your computer but the A , X , and Y are by far the most often used and easier to understand for beginning ML programmers . . Throughout this article ,  you will learn the use of some ML instructions known as mnemonics . .  These allow us humans to easier relate to ML programming than the way the microprocessor would . . One of the most common instructions you will be using is LDA . . This simply means LoaD Accumulator . . Here is an example . . 

 LDA #$00

 This command simply placed the vallue 0 in the A register . . The # preceding the value tells the computer that you wish to use the immediate mode of LDA . . This means that the actual value ( $ 0 0 ) was loaded into the Accumulator . . The same thing can be accomplished from BASIC like this . . 
 
POKE780,0

 Location 7 8 0 is the storage area for the A register . . The immediate mode uses two bytes of computer memory . . One for the opcode and one for the operand . . There is another addressing mode which you may use LDA with . . That is the absolute mode . . In this , you instead load the Accumulator with the contents of a memory location . . This mode is specified by not including a #  in front of the operand . . 

 LDA $FB

 The contents of $ FB ( 2 5 1 decimal ) are loaded into A with this instruction . .  So , if $ FB contained $ 8 0 , A would then contain $ 8 0 as well after this command was executed . . Accomplishing this task in BASIC would be as easy as this . . 

 POKE780,PEEK(251)

 The absolute mode uses up three bytes of memory . . One for the opcode , and one each for the low and high bytes of the address ( if it was not in zero page as in the example which would use only two bytes . . ) Now that you know how to load A , let ' s put what you loaded into there to work . . We all know that location  5 3 2 8 0 holds the current color code of  the border and 5 3 2 8 1 holds the color for the background , right ? Using the STA ( STash Accumulator in ) command , you place the contents of A wherever in memory you wish . . See if you can see what this short ML routine does ..

 LDA #$00
STA $D020
STA $D021

 It may seem a little tricky at first seeing as how I used hexadecimal numbers ( symbolized by the $ sign ) for the memory locations . . But all it does is put a 0 in both location 5 3 2 8 0 and  5 3 2 8 1 . . Effectively changing the screen black . . If you wished to do the same in BASIC you would only need to POKE both locations with a 0 . . Is this easy or what ? You should probably be getting a VERY basic idea of how ML works by these few examples .. Now , to cut this already overly long article short ,  here ' s a few new ones you should be able to figure out which I will quickly define for you . . 
 
LDA #$01
STA $FC
LDX #$80
LDY $FC
STX $28A
STY $286

 The first two you already know . . It puts $ 0 1 into A and then into location  2 5 2 . . Then you come across a new command . . LDX means LoaD X . . The example puts the value $ 8 0  ( in decimal that ' s 1 2 8 ) into X . . You should  ( hopefully ) be able to figure out what LDY means . . That command puts the contents of location $ FC ( 2 5 2 ) into Y . . Which was $ 0 1 from the above STA command . . After that , STX ( STash X in )  puts the contents of X into location  $ 2 8 A . . Then finally , we use STY to store what we had in Y ( $ 0 1 ) into location $ 2 8 6  ( 6 4 6 ) . . Now , if you examine all these closely you ' ll realize you actually modified your computer in several ways by using these commands . . Let ' s assume our screen is still black from the example of the STA commands above . . So , after storing Y with $01 by stashing it into $ FC with the STA command , you change the color of the screen character . . Remember ,  POKEing location 6 4 6 will do that in BASIC . . You also loaded X with $ 8 0 and  then stashed it into $ 2 8 A ( 6 5 0 ) . . This location determines whether or not all the keys you press repeat or not . . A  $ 8 0 or 1 2 8 here will make it so that that happens . . Now after all this , you have a black screen with a white cursor that repeatedly prints everything you press . . Neat , huh ? This is just a VERY small sample of what you can do with machine language ! The applications are endless and it is much more efficient and faster than BASIC which could have accomplished the same thing if you so chose . . Hopefully , ML is not as mysterious to you who thought it was some foreign language before this quick tutorial . . It is difficult at first ,  but it becomes as easy as BASIC and better with practice . . So , until the next newsletter , see what other things you can do with these kinds of instructions . . You ' ll learn even more next time , though , there ' s a LOT to cover still . .