VGA example - passing data between code

retromicro

Hi

I'm interested in passing data from a SPIN array into PASM so I've been looking at some example code.

Please correct me if I'm wrong here.

In one of the VGA examples, a block of characters (longs) is defined like so -

long screen[cols*rows/4]

It looks like the address of these characters (longs) is passed through to the main VGA set up routine like so -

vgatext.start(16, @screen, @colours, @cx0, @sync)

PUB start(BasePin, ScreenPtr, ColorPtr, CursorPtr, SyncPtr) : okay | i, j

This leaves ScreenPtr containing the address of the character block.

This is soon followed up with -

'implant pointers
longmove(@screen_base, @ScreenPtr, 3)

Not sure why they would do this, especially choosing to move 3 longs. Further down, screen_base looks to be the address of the next character to be rendered but why not just use ScreenPtr ?

Once inside the main video rendering PASM, we have -

mov screen_ptr,screen_base 'reset screen pointer to upper-left character

following soon after by -

:column rdbyte z,screen_ptr 'get character from screen memory
.

Intermediate code
.
add screen_ptr,#1 'increment screen memory address

It looks like screen_ptr is set up from screen_base as a destructible pointer into the original character block and this is incremented each time in the loop ready to get the next character.

However, I've got some questions about this -

RDBYTE is used to read byte of main memory but isn't the character block declared in the SPIN in Cog RAM ?

This almost looks like indexed addressing but without the self-modifying part.

I've got my own simple example with extracts below -

long inverse

inverse := 255

longmove(@inverse_flag, @inverse, 1)

The PASM code simply has the below and the value assigned in SPIN is seen by the PASM.

XOR led_byte, inverse_flag ' Temporary inversion to test parameter passing

inverse_flag LONG 0

This looks like a simpler way than the VGA example for PASM to access data defined in SPIN.

So, after all the above rambling, what's the best way for a PASM loop to access a block of data as set up in the top SPIN example ?

Thanks

kwinn

There are several ways to pass parameters from spin to the cog. You can use the "par" register, place it in the "dat" section that gets loaded to the cog, or use the "par" register as a pointer to hub and have the pasm code read the data from the hub. All depends on what you are doing.

For vga character display the cog needs to know what pins to use for the signals, the start address of the bytes to display, the address of a lookup table to convert each character to pixels for display, and a color palette for the background/foreground colors.