Reminder:

This is not to support cheating. It is merely to document the LED on a hardware/software level.
This can be used for custom OS's which want to integrate a proper examination mode that abide by the rules.
Cheating can get you banned from any examination center.

Casio Graph LED documentation

The OS used in my disasembly is: version 2.05.

The LED is controlled by what is called the "C-specification module" or "CMOD" in Casio's emulator files.
See Yatis' documentation for more information.
Two addresses are used:

0xa44c0020 (CMOD.DDCK_CNTR)
0xa44c0000 (CMOD.DDCLKR0)

CMOD.DDCK_CNTR

CMOD.DDCK_CNTR is the external CLK1 setting and it is an 8 bit address.
If set incorrectly it can cause the calculator to become unstable.

The setting used to control the LED is 0x4 (00000100).
And is set to 0x5 when not used. (00000101)

CMOD.DDCLKR0

CMOD.DDCLKR0 is a 16 bit address.
When the DDCK_CNTR is correctly set, we can adjust the brightness, flicker or enable/disable the LED.

Bit	Name (made up)	Mode	Description
15	???	???	Is always set to 1 by OS, but behaviour is unkown.
14	LED_toggle	R/W	Enable/disable bit for LED
13	LED_ftime1	R/W	Light time for flicker?
12	LED_ftime2	R/W	Light time for flicker?
11	LED_ftime3	R/W	Light time for flicker?
10	LED_ftime4	R/W	Light time for flicker?
9	LED_ftime5	R/W	Light time for flicker?
8	LED_ftime6	R/W	Light time for flicker?
7	LED_ftime7	R/W	Light time for flicker?
6	LED_ftime8	R/W	Light time for flicker?
5	LED_ftime9	R/W	Light time for flicker?
4	LED_ftime10	R/W	Light time for flicker?
3	LED_brightness1	R/W	Sets the brightness of the LED? (unsure)
2	LED_brightness2	R/W	Sets the brightness of the LED.
1	LED_brightness3	R/W	Sets the brightness of the LED.
0	LED_brightness4	R/W	Sets the brightness of the LED.

The brightness, and flicker is kind of a weird one.
They don't seem to be increasing values, eg brightness from 1-15 (or 7).

Anyways, here are 3 values used by the LED test in diagnostic mode:
Minimum - 011
Middle - 001
Maximum - 101

Usage in code

Thanks to the FXOS dissasembler.
A snippet of the LEDon function used in Diagnostic mode:

mov.l   r14, @-r15
sts.l   pr, @-r15
mov.l   0xa4050100 PFC.PACR, r5
mov.w   0x000003fc, r14
mov     #1, r2
mov     #1, r4
shll8   r2
mov.w   @r5, r1
and     r14, r1
mov.w   r1, @r5
add     #78, r5
mov.l   0xa44c0020 CMOD.DDCK_CNTR, r14
mov.w   0xfffffcff, r1
mov.w   @r5, r7
and     r1, r7
or      r2, r7
mov.w   r7, @r5
mov.b   @r14, r0
mov.l   0x8003ffdc, r7
and     #-2, r0
jsr     @r7
mov.b   r0, @r14
bsr     <0x800427e8>
mov     #-1, r4
mov.l   0x801d1eb8, r1
shll    r0
mov.l   0xa44c0000 CMOD.DDCLKR0, r2
mov     #1, r4
mov.l   0x8003ffdc, r5
mov.w   @(r0,r1), r6
jsr     @r5
mov.w   r6, @r2
mov.b   @r14, r0
or      #1, r0
mov.b   r0, @r14
lds.l   @r15+, pr
rts
mov.l   @r15+, r14

Simplified, and unrelevant commands removed:

#DDCK_CNTR #1
mov.l   0xa44c0020 CMOD.DDCK_CNTR, r14  #Move 0xa44c0020 to r14
mov.b   @r14, r0                        #Move value at 0xa44c0020 to r0
and     #-2, r0                         #AND r0 with decimal -2 (11111110)
mov.b   r0, @r14                        #Move result (r0) to memory location 0xa44c0020

#DDCLKR0
bsr     <0x800427e8>             #Move to function
|
-----> mov     #1, r0                   #Move decimal 1 into r0
mov.l   0x801d1eb8, r1                  #Move 0x801d1eb8 into r1
shll    r0                              #Bitwise-Shift R0 to the left
mov.l   0xa44c0000 CMOD.DDCLKR0, r2     #Move 0xa44c0000 to r2
mov.w   @(r0,r1), r6                    #Move value at location 0x801d1eb8+0x2 to r6 (1100000000000011)
mov.w   r6, @r2                         #Move r6 to memory location 0xa44c0000

#DDCK_CNTR #2
mov.l   0xa44c0020 CMOD.DDCK_CNTR, r14  #Move 0xa44c0020 to r14
mov.b   @r14, r0                        #Move byte value at 0xa44c0020 to r0
or      #1, r0                          #OR r0 with decimal 1 (00000001)
mov.b   r0, @r14                        #Move result (r0) to memory location 0xa44c0020

Now in C code:

char*CMOD_DDCK_CNTR=(void*)0xa44c0020;      //Word
short*CMOD_DDCLKR0=(void*)0xa44C0000;       //Byte

void TurnONLED() {
	*CMOD_DDCK_CNTR &= 0xFE;            //11111110
	*CMOD_DDCLKR0 = 0xc003;             //1100000000000011
	*CMOD_DDCK_CNTR |= 0x01;            //00000001
}

We can do the opposite for turning the LED off:

void TurnOFFLED() {
	*CMOD_DDCK_CNTR &= 0xFE;            //11111110
	*CMOD_DDCLKR0 = 0x0;                //0000000000000000
	*CMOD_DDCK_CNTR |= 0x01;            //00000001
}

However the LED will automatically be turned off.
What code is responsible for turning it off automatically, I yet have to find.

To circumvent this issue we can do the following:

SetAutoPowerOffFlag(); //Syscall 0x48D
Cursor_SetFlashOff(); //Syscall 0x812

This is what is used in the beginning by diagnostic mode. I've yet to find how the examination mode circumvents this issue.
But it will break the USB timer, so you wont get a popup anymore when connecting your calculator.

About

Documentating the behaviour of the LED on the Casio Graph 35+e

MIT License