PROJECT: Debug And Polish (DAP1)

For our debug to work with any given code we need to preserve everything that we may be using; part of this is to save the contents of our registers.

To do this we can set up a local stack to save the contents of our register while we use them with the instructions:

PUSH BP
MOV  BP, SP

Once we set up a local stack, we can push all of our registers onto this new stack, while still having access to the previous stack with some manipulations.

Once you have done everything you need to, you need to restore your registers and stack.
For the registers, simply pop them off the stack in reverse order, i.e if you pushed 0-13, you will pop 13-0. The stack can be just as simple as that, or more difficult depending on your approach. The simpler approach is to do the reverse of creating a local stack, that is:

MOV  SP, BP
POP  BP

Along with preserving your register you need to preserve your texture and region, this should be done after preserving at least one of your registers, as we need to use a register to get the data to preserve.
Once we have preserved our registers, simply use the 'IN' instruction to get information from the GPU port like this:

IN   R0, GPU_SelectedTexture
PUSH R0
IN   R0, GPU_SelectedRegion
PUSH R0

And just like our registers, we need to pop them in reverse order to restore them when we're done:

POP  R0
OUT  GPU_SelectedRegion, R0
POP  R0
OUT  GPU_SelectedTexture, R0

The AND op code will do an and comparison of two values

mov  R0,  0x00F00000
mov  R1,  0x12689649
and  R0,  R1

This code will leave us with 0x00600000 because every other value was put into and with a zero.

A shift left can be done with the op code SHL

mov  R0,  0x00F00000
mov  R1,  4
shl  R0,  R1

This code will give R0 the value 0x0F000000 because the original value was left-shifted by 4 bits.

There isn't an op code provided by Vircon32 for right shifts, so we must shift left by a negative value.

mov  R0,  0x00F00000
mov  R1,  -4
shl  R0,  R1

This code will give R0 the value 0x000F0000 because we shifted the original value to the left by -4 bits.

SET DATA
SET X
SET Y
SET POSITION TO LEFTMOST NIBBLE
UNTIL WE HAVE DISPLAYED ALL NIBBLES:
    OBTAIN NIBBLE AT POSITION
	SHOULD NIBBLE BE GREATER THAN OR EQUAL TO TEN:
	    ADD SEVEN TO ITS VALUE
	ADD FORTY EIGHT TO ITS VALUE
	DISPLAY NIBBLE AT X, Y
	INCREMENT X ACCORDINGLY
	ADJUST POSITION TO NEXT NIBBLE TO THE RIGHT
REPEAT

In your code you will want to put

%include "debug.s"

at the very end of your code. THIS IS VERY IMPORTANT. If you include debug.s at the top of your code, the hexadecimal values may print but the remainder of the program in which you called __debug may not function as expected.

Beyond getting your values to be displayed you also need to include “0x” to signify that the number you are displaying in hexadecimal. It is important to know that the 0 and x have a number assigned to them in ASCII code which are as follows:

ASCII code for 0 --> decimal 48, hexadecimal 0x30
ASCII code for x --> 120
ASCII code for [ --> 91
ASCII code for ] --> 93
ASCII code for : --> 58

Knowing this, we can now select where we want to display these values by simply calling the following:

    mov R0, 48 ; ASCII code for 0 is moved into RO
    out  GPU_SelectedRegion, R0 ; Selecting region
    out  GPU_DrawingPointX, R2  ; Providing X axis location
    out  GPU_DrawingPointY, R3  ; Providing Y axis location
    out  GPU_Command, GPUCommand_DrawRegion ; Displaying
    iadd R2, 9 ; Spacing logic

We need to get a nibble on it's own, to do this we can use a bit mask

A mask only allows certain bit positions to be either 1 or 0, and forces everything else to be 0

This is done with the and operator

In order to get the most significant nibble we would want a mask of 0xF0000000 (0b11110000000000000000000000000000)

mov   R1,   0xF0000000  ; Creating the mask
and   R1,   R0          ; R0 is the original value

To get the second most significant nibble, instead of creating a new mask by hand, we can shift the old mask over by a nibble ever time we loop

We should have a register keep track of our loop count

mov   R2,   R3          ; R3 is the loop count
imul  R2,   -4          ; Left shift bits per nibble

mov   R1,   0xF0000000  ; Creating the mask
shl   R1,   R2
   
and   R1,   R0          ; R0 is the original value

Because we already have the debug function, we can use that to display all of the hex values needed

The format is identical for every address, and goes as the following: [address]:value

We can print the brackets and colon manually and leave the address and values to the debug function

mov   R3,         91          ;ASCII code for '['
out   GPUREGION,  R3
mov   R3,         0           ;Xpos
out   XDRAWINGP,  R5
out   GPUCOMMAND, DRAWREGION  ;Draw

Repeat this for ']' and ':' changing the Xpos and accounting for the address width

Utilize the debug function to print the address and value as shown in the following

push  R2                      ;R2 is the current address
mov   R3,         10          ;Xpos
push  R3
push  R4                      ;Ypos
call  _debug

mov   R3,         [R2]        ;Get the value
push  R3
mov   R3,         130         ;Xpos
push  R3
push  R4                      ;Ypos
call  _debug

All of this code should be run for every address, stepping from the start value to the end value

We will need to setup a loop with an exit condition

MOVE THE STARTING ADDRESS INTO A DEDICATED REGISTER (current address)
UNTIL WE HAVE PASSED THE FINAL ADDRESS:
   TEST IF THE CURRENT ADDRESS IS GREATER THAN THE FINAL ADDRESS
      SHOULD IT BE GREATER SKIP TO AFTER THIS LOOP
   DISPLAY THE ADDRESS AND VALUE
   INCREMENT THE YPOS
   INCREMENT THE ADDRESS
REPEAT