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base:machine_language_tutorial_part_2 [2015-05-04 04:42]
karmic [Machine Language Tutorial Part 2 - Memory Manipulation]
base:machine_language_tutorial_part_2 [2015-11-01 23:26] (current)
karmic [The Example]
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 +====== Machine Language Tutorial Part 2 - Memory Manipulation ======
 +The 6510 has three registers that can all hold 8 bits of data (this is why the C64 is called an 8-bit computer): A, X, and Y.
 +The proper name for the A register is the accumulator because of its ability to do math, and X & Y are called index registers.
 +You cannot transfer data directly between two memory addresses, so these must pass through the registers.
 +To load a byte into registers, use these commands:
 +<​code>​LDA - load into A
 +LDX - load into X
 +LDY - load into Y</​code>​
 +So the "LDA $1234" example from last part would load whatever was in $1234 into A.
 +To store registers into memory, use these commands:
 +<​code>​STA - store A into
 +STX - store X into
 +STY - store Y into</​code>​
 +So the "STA $4321" example from last part would store whatever was in A into $4321.
 +To transfer data between registers, use these commands:
 +<​code>​TAX - transfer A to X
 +TAY - transfer A to Y
 +TXA - transfer X to A
 +TYA - transfer Y to A</​code>​
 +These commands have no operand, so just "​TAX"​ would be sufficient.
 +To increment or decrement registers or memory, use these commands:
 +<​code>​INC - increment memory
 +INX - increment X
 +INY - increment Y
 +DEC - decrement memory
 +DEX - decrement X
 +DEY - decrement Y</​code>​
 +So "INC $1111" would increment $1111. INX and INY have no operand. You'll use INX/​DEX/​INY/​DEY a lot, so remember them!
 +===== A Little Tangent =====
 +Alright, so we have a question that still needs to be answered: where do we put our programs?
 +The cassette buffer, $033C-$03FB is a good spot to put these short example programs.
 +But in the future, you're going to need to use different spots of memory.
 +There'​s a completely free block of RAM in $C000-$CFFF,​ but that's still only 4K.
 +$0800-$9FFF is the normal spot if you're not using BASIC at all, but usually you're going to include a SYS-line, so you'll need to start at $080D if so. If you have the BASIC and Kernal-ROMs turned off entirely, you've also got almost all of $A000-$BFFF and $E000-$FFFF. Don't use $D000-$DFFF,​ that's where the I/O (SID, VIC, CIA) lies.
 +===== The Example =====
 +Objective: We want to get $46 into A, transfer that to X, increase it, put it back into A, and then store that into $CFFF.
 +The first problem comes right away: how do we get the specific value $46 into A? We can't do LDA $46, or else it would load from the zeropage $46.
 +The solution is that we have to add a number sign preceding the value. This is called immediate addressing. Using a full address (LDA $FFFF) is called absolute addressing, and zeropage access (LDA $FF) is called zeropage addressing. We'll learn about more addressing modes later.
 +So type this into the monitor:
 +<​code>​A 033C LDA #​$46</​code>​
 +Remember what this means? It means that we're Assembling LDA #$46 to $033C.
 +So now we see that the monitor has prepared the next line, now all we must type to transfer is:
 +And now we increment it:
 +Put it back into A:
 +And store it (absolute addressing):​
 +<​code>​STA $CFFF</​code>​
 +But now there'​s another issue: how do we tell the computer to stop execution? If we didn'​t,​ it would just go into a random series of commands that would inevitably crash it.
 +To tell the computer to quit this program, we use:
 +This breaks the program and jumps to the address at $0316. In this case, it returns to the monitor.
 +Note: Using BRK to end a program is generally a bad idea for actual programs. We're only using it in this example.
 +We're done, now just press return on the next line to exit assembly mode.
 +To start the program, we could do two things: go back to BASIC and type SYS828 ($033C in decimal), or type in the monitor:
 +<​code>​G 033C</​code>​
 +This command (goto) changes the Program Counter so the next thing it executes is your program (the instruction at $033C).
 +Now type R into the monitor to view the registers, and type M CFFF. See anything different?
 +End of part 2...
base/machine_language_tutorial_part_2.txt ยท Last modified: 2015-11-01 23:26 by karmic