Q How do these 2 examples have the same effect as putting red in the background colour?
Why is
dc.w $180,$600
the same as:
dc.w $180
dc.w $600
The first example sets red into the system register for background colour (DC.W DESTINATION:SOURCE). The second example is equivalent - how? Does the compiler 'know' that $180 within a copperlist is a special register and just accepts the next DC.w it comes across as the value?
If you put the following in the copper list at the end, no complaints, code still runs. Is it putting $600 into location $700? What is the difference here?
dc.w $700
dc.w $600
The notes mention the equivalence here
First, watch this video
An assembled program is loaded into memory starting from one starting address. Each memory address can can be thought of as a box that can hold a byte of data. See this example using h.w (hex dump of memory by size WORD). Notice on the left column how the memory address increments.
It's useful to use the h (hex dump) command to study this. The whole program is represented by each memory location laid out one after the other with its execution starting at the base address. If you start your program with a label such as
Init:
move.l 4.w,a6 ; Execbase in a6
jsr -$78(a6) ; Disable multitasking
lea GfxName(PC),a1 ; Address of the name of the lib to open in a1
jsr -$198(a6) ; OpenLibrary, EXEC routine that opens
You can use h.w Init to show the contents of memory from the address of the Init label:
dc writes directly to the memory sequentially. This is why the above examples are the same. To prove it start with the code:
BAR:
dc.w $7907,$FFFE ; WAIT - wait for line $79
dc.w $180,$600 ; COLOR0 - I start the red zone: red at 6
Then use h.w BAR to show
Then change the code to
BAR:
dc.w $7907,$FFFE ; WAIT - wait for line $79
dc.w $180 ; COLOUR0
dw.w 600 ; red at 6 to start
Then use h.w BAR to show
Notice it is the same data in the memory location of BAR whichever method we choose. There is still some magic to me in that the instruction at memory location $00000970 being run by the copper results in register $dff180 taking the value of $0600 (red).
You can see FFFF FFFE in the pictures, which we know when run in the copper means the end of the copper list. This is because my code for these exmaples is
BAR:
dc.w $7907,$FFFE ; WAIT - wait for line $79
dc.w $180
dc.w $600 ; COLOR0 - I start the red zone: red at 6
dc.w $FFFF,$FFFE ; END OF COPPERLIST
end
Q What exactly is stored at each memory location? How is it organised?
Each memory location can store 1 byte. The below picture show using the memory command to inspect the memory contents for the label BARRA m BARRA
You can see BARRA starts are memory location $00016EC8 and it contains a byte with value of 79. Each subsequent byte shown on the same line is the value stored at the next memory location. So $00016EC9 contains 1 byte with value 07 and so on.
The debugger is showing us the memory contents 16 bytes per line which is why the second line of the output is labelled on the left with address $00016ED8. I label each memory location on the first line to illustrate this.
If we look at the code where BARRA is defined, you can see the memory output matches what we would expect
Q Dispaying an image correctly - were am I going wrong with my example?
For example 4 code here
Images used listed below (sizes seem messed up):
AGAiff exported 3 bit plane raw
The colours used to render the image are hard coded in the copper list in the code. See this line onwards