MACHINE LANGUAGE TUTORIAL PART IV The CMP command CMP stands for CoMPare accumulator. It h

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*************************************** * MACHINE LANGUAGE TUTORIAL PART IV * *************************************** The CMP command CMP stands for CoMPare accumulator. It has an immediate mode. In immediate mode, what happens is this: the value of the arguement is subtracted from the contents of the accumulator and this result is dicarded except for the effects on the zero, negative and carry flags. English translation immediately following. We'll take it slowly. The value of the arguement is the byte following the operator. This value is subtracted from the contents of the accumulator. Say the arguement is $40 and the accumulator holds $60. $40 is subtracted from $60 and you get $20. Now supposing the arguement value is greater than the value in the accumulator, that is $60 is subtracted from $40. Doing this algebraicly, you would get -$20, but the accumulator can only hold numbers from zero to 255. So what happens is that the microprocessor adds $100 to it. -$20+$100=$D0. And from this number, the flags take thier cues. By the way, this resulting number is thrown out and forgotten about. What are flags?? The flags that were mentioned live in the status register. Also called the 'p' register. This register is an 8-bit register and a flag is one of these bits. However, even though there are eight bits in the register, there are only seven flags. These flags are: 1> Carry 2> Zero 3> Interupt disable 4> Decimal mode 5> Break command 6> Overflow 7> Negative. For now, we will deal only with the zero, and negative flags. The zero flag is set to 1 whenever a zero is loaded, stored, or gotten as a result in an arithmatic command, such as the CMP. The negative flag is set to one whenever a negative number is loaded, stored, or gotten as a result in an arithmatic command. This machine of ours defines a negative number as any number that has its highest bit set to 1. That is, any number greater than $7F. Branching. In essence, what the branching statements do is this. They check a specific flag and then depending on whether that flag is a 1 or a zero, go to a location specified by the arguement. For example, BEQ branches when the zero flag is set to 1. (If the flag is a 0, program flow continues on with the statement following the branch command. Let's take a look at a short little program. 300:A2 00 LDX #$0 302:BD 11 03 LDA $0311,X 305:C9 00 CMP #$0 307:F0 07 BEQ OUT 309:20 ED FD JSR $FDED 30C:E8 INX 30D:4C 02 03 JMP $0302 310:60 OUT RTS 311:C4 D2 AE ... (This is hex representation of text and ends with a $0) This program will print out whatever the text says (text is at $311 and is in ASCII chrs) using a ROM routine at $FDED which prints characters onto the screen. The text must end with a $0 and be less than 255 bytes long, otherwise you will either hang the system or fuck it up royally. Even though there are quite a few new commands in the program, we will only focus on the role of the BEQ command. The second byte of the command (BEQ is a 2-byte command) is the arguement and determines where the program branches to, if it branches. The way which the address of the branch is determined is this. The M.P.U takes the address of the next command after the branch statement (in our program above, this address would be $309) and adds the value of the arguement to it. In our program, the arguement for the branch statement is $7. $7+$309=$310. Which is what we want to happen. But, all number from $80 up are negative! So if the arguement had been $F8, $F8 is equal to -$8 and $309-$8=$301. The program would have jumped to $301. BNE BNE is a branch command that branches when the Z flag=0, that is a non-zero number is stored, loaded or gotten in an arithmetic operation. This command works in exactly the same way as the BEQ to generate the 'branch-to' addresses. A word on the other commands. In the program, there were a lot of new commands used, i will cover them in the near future, but just to give you an idea, here are some quick defenitions. JMP: this is much like a BASIC 'goto'. JSR: this is much like a BASIC 'gosub'. INX: increment X by 1. (add 1 to the value in the X register) RTS: this is a general 'end-of program' statement. there are some better uses which we will cover. Well, that's all folks! *************************************** * * * Dr. Firmware's M.L * * tutorial * *************************************** * * * TESTAMENT:(514)-332-6852 * * TRANSFERS AE:(514)-738-1247 * * * ***************************************


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