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*************************************** * * * PART VII - ASSEMBLERS * * WRITTEN BY DR. FIRMWARE * * * *************************************** Assemblers are used for easily writing up code from mnemonics to hex. To do this by hand is tedious, to say the least, and eventually one will make an error here or there. Mnemonics are the codes that we have been using, like 'LDA'. Since these do not signify the addressing mode, there is a set of symbols that are normally used. To indicate immediate addressing mode, we put a '#' in front of the arguement. To indicate absolute addressing mode, we just put the address. To indicate indexed absolute mode, we put the base address followed by a comma and the indexing register. Here is a short list of the conventions: LDA #$00 -IMMEDIATE LDA $0000 -ABSOLUTE LDA $0000,X -ABSOLUTE IND. X LDA $0000,Y -ABSOLUTE IND. Y LDA $00 -ZERO PAGE LDA $00,X -ZERO PAGE,X LDA $00,Y -ZERO PAGE,Y LDA ($00,X) -INDIRECT,X LDA ($00),Y -INDIRECT,Y JMP ($0000) -INDIRECT INX -IMPLIED ASL A -ACCUMULATOR The modes will be fully explained further down. Here'S a simple program in assembly language: (1) (2) (3) (4) ORG $300 Start at $300 COUT EQU $FDED COUT stands for $FDED LDX #$0C Load X with length. LOOP LDA TEXT,X Load A with a chr. JSR COUT Gosub chr output at $FDED DEX Decrement X by 1. CPX #$00 Is it zero yet? BNE LOOP If not goto to 'LOOP' RTS Else end. TEXT ASC 'DR. FIRMWARE' - ASCII chrs for my name. The columns denoted by the numbers in brackets are as follows: (1) label field, (2) operator field, (3) arguement field, and (4) comment field. Labels. Labels are used in assembly language to simplify things. The label 'PLOTIT' means a lot more than $27A5. Note that labels are all one word, no spaces. In this program, the label 'LOOP' is used to denote a specific place in the program. In the branch statement, 'LOOP' is refered to, and when the program is assembled, the address in memory where 'LOOP' will be is the address the argument the statement will use. The operator field. This is where the mnemonics are. The main part of the program is here. However, you might have noticed the 'ORG' and the 'ASC'. These are 'psuedo- ops'. These pseudo-ops tell the assembly program needed information such as the address where the program is supposed to run. There are many pseudo-ops, and since each assembly program has thier own, it would be hard to cover all of them. So, refer to any manuels that you've copied with your software. Arguement field. This field is where the arguements for the operators are, if there need to be any given. The arguments need not to be hex numbers any more. One can use labels for everything, if it pleases you. But in general, since main point of assembly programs is to let the programmer program and not mess around with (yucky) hex numbers, labels in this field seem to be the way to go. Comment field. This field is to help narrate your program, that is, to help someone who is reading it (including yourself at times, i'm sure). Of course one can put things like editorials or dirty msgs here, but each to his own. In this column, i will be using a nice mix of psuedo-ops and comments, so, if this program doesn't work as typed, sue me. Ok, with that out of the way, here is a description of the previously mentioned addressing modes. Zero page. Zero page is somewhat special because the MSB of all the bytes is $00. For this mode, there is only one arguement byte. This byte is the LSB of the address and you will get addresses like $0045. When indexing zero page with either X or Y, the resulting address is always smaller than $100. For example, LDA $45,X when X holds $FF will read address $44 and put it in the accumulator. The logic goes thus: $45+ $FF= $144. Because the result is greater than $100, the one at the front is dropped and all you have left is $44. JMP. This is a goto-like command in m.l. and can be considered as such. The command has 2 argument bytes and these represent the address where program execution will continue in the form LSB MSB. Note the address to jump to is backwards just like the LDA command in absolute mode. Indirect jump. The indirect jump is variation on the JMP, such that the argument forms an address from where the actual 'jump to' address is found. (Both in MSB LSB form.) Suppose there was such an incident: 300: JMP ($800) . . 800: $00 $20 ($800 Contains $00 and $801 contains $20) From $300, the argument gives $800. The program goes and gets $800 and $801 and re-arranges them to give $2000. Then the program jumps to $2000 and continues execution. A very useful command at times. Well, unfortunately the indirect commands will have to wait 'til next time. *************************************** * DR. FIRMWARE CAN BE REACHED ON THESE* * BOARDS: 514-738-6576 TRANSFERS * * 514-744-4108 APPLE ENCH. * ***************************************


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