Syscall:   0x24a Keyboard_GetPressedKey()
Objective: Determine how to wait for the keyboard when reading port data
           registers to avoid breaking the keyboard when overclocking.
           Gather some information on ghosting.

Information found:
* ...

=======================

# Just leave it to <3f9d6>. NB: This is not a syscall.
 3fc48:  aec5	bra	<3f9d6>
 3fc4a:  0009	nop

=======================

# Make some stack space for memory-mapped keyboard representation.
# Disable keyboard interrupts by calling subroutine (see below).
 3f9d6:  2fe6	mov.l	r14, @-r15
 3f9d8:  4f22	sts.l	pr, @-r15
 3f9da:  7ff0	add	#-16, r15
 3f9dc:  d128	mov.l	#0x8003e264, r1
 3f9de:  410b	jsr	@r1
 3f9e0:  1f43	mov.l	r4, @(12, r15)

# Loads 12 bytes of KEYSC data to the stack.
# Second subroutine call, with parameters : stack top and {short *matrixcode}.
# Checks whether the key given in the matrixcode was pressed.
 3f9e2:  d528	mov.l	#0xa44b0000, r5
 3f9e4:  6451	mov.w	@r5, r4
 3f9e6:  2f41	mov.w	r4, @r15
 3f9e8:  64f3	mov	r15, r4
 3f9ea:  8551	mov.w	@(2, r5), r0
 3f9ec:  81f1	mov.w	r0, @(2, r15)
 3f9ee:  8552	mov.w	@(4, r5), r0
 3f9f0:  81f2	mov.w	r0, @(4, r15)
 3f9f2:  8553	mov.w	@(6, r5), r0
 3f9f4:  81f3	mov.w	r0, @(6, r15)
 3f9f6:  8554	mov.w	@(8, r5), r0
 3f9f8:  81f4	mov.w	r0, @(8, r15)
 3f9fa:  8555	mov.w	@(10, r5), r0
 3f9fc:  81f5	mov.w	r0, @(10, r15)
 3f9fe:  bd30	bsr	<3f462>
 3fa00:  55f3	mov.l	@(12, r15), r5

# Loads IPRF address to r6, sets the keyboard interrupt priority to 13 and
# performs a third subroutine call : enables keyboard interrupts (curiously
# the interrupt priority is set two times).
# Also saves the last subroutine result (key pressed) to r14.
 3fa02:  d621	mov.l	<3fa88>(#0xa4080014), r6
 3fa04:  9113	mov.w	<3fa2e>(#0x0fff), r1
 3fa06:  9213	mov.w	<3fa30>(#0xd000), r2
 3fa08:  6e03	mov	r0, r14
 3fa0a:  6761	mov.w	@r6, r7
 3fa0c:  2719	and	r1, r7
 3fa0e:  272b	or	r2, r7
 3fa10:  d212	mov.l	#0x8003e26c, r2
 3fa12:  420b	jsr	@r2
 3fa14:  2671	mov.w	r7, @r6

# Restores the 'key pressed' result to r0, restores saved data and returns.
 3fa16:  60e3	mov	r14, r0
 3fa18:  7f10	add	#16, r15
 3fa1a:  4f26	lds.l	@r15+, pr
 3fa1c:  000b	rts
 3fa1e:  6ef6	mov.l	@r15+, r14

=======================
Subroutine call at <3f9de>.
- Denies keyboard interrupt in IMR5
- Disables keyboard interrupt in IPRF
=======================

# Just leave it to <3e278>.
 3e264:  a008	bra	<3e278>
 3e266:  0009	nop

=======================

# Writes 0x80 to IMR5.
 3e278:  d478	mov.l	<3e45c>(#0xa4080094), r4
 3e27a:  e180	mov	#-128, r1
 3e27c:  955b	mov.w	<3e336>(#0x0fff), r5
 3e27e:  2410	mov.b	r1, @r4

# Clear the four upper bits of IPRF.
 3e280:  7480	add	#-128, r4
 3e282:  6241	mov.w	@r4, r2
 3e284:  2259	and	r5, r2
 3e286:  000b	rts
 3e288:  2421	mov.w	r2, @r4

=======================
Subroutine call at <3f9fe>
- Detects whether a specific key is pressed using the KEYSC data.
- Returns 1 if the key is pressed, 0 otherwise.
=======================

Stack:
    r4 (bottom stack address)
    {short *matrixcode}
    ------  Bottom stack address.

# Pushes {short *matrixcode} on the stack and on top, pushes r4.
# Loads the key column to r2 and replaces r5 by #1 (r5 will then be shifted to
# be used as a mask in a KEYSC word).
 3f462:  7ff8	add	#-8, r15
 3f464:  1f51	mov.l	r5, @(4, r15)
 3f466:  6250	mov.b	@r5, r2
 3f468:  e501	mov	#1, r5
 3f46a:  51f1	mov.l	@(4, r15), r1
 3f46c:  622c	extu.b	r2, r2
 3f46e:  2f42	mov.l	r4, @r15

# Loads the key row to r0.
 3f470:  8411	mov.b	@(1, r1), r0
 3f472:  600c	extu.b	r0, r0
# Shifts it right within r6 and tests the row's lower bit.
 3f474:  6603	mov	r0, r6
 3f476:  4621	shar	r6
 3f478:  c801	tst	#1, r0

# Shift r5 by its column number.
# If the row's lower bit was 1, then also shift r5 by 8 (because there are two
# rows for each word).
 3f47a:  8d01	bt/s	<3f480>
 3f47c:  452d	shld	r2, r5
 3f47e:  4518	shll8	r5

# Loads the bottom stack address to r1.
 3f480:  61f2	mov.l	@r15, r1
# Erases the even/odd bit of the row in r6 (because the KEYSC data is read in
# words so we don't want to get on a byte offset, and the lower bit has
# already been handled when shifting r5).
 3f482:  4600	shll	r6

# Loads the final mask into r2.
 3f484:  625d	extu.w	r5, r2
 3f486:  6063	mov	r6, r0
# Tests whether the key represented by the parameter matrixcode is pressed.
 3f488:  041d	mov.w	@(r0, r1), r4
 3f48a:  2428	tst	r2, r4
# Returns the KEYSC bit, that is 1 if the key was pressed, 0 otherwise.
 3f48c:  0029	movt	r0
 3f48e:  ca01	xor	#1, r0
 3f490:  000b	rts
 3f492:  7f08	add	#8, r15



-----------------------------------
Subroutine call at <3fa12>
- Sets the keyboard interrupt priority to 13.
- Clears the keyboard interrupt mask.
-----------------------------------

    Calls redirects to 8003 e28a.

 3e28a:  d475	mov.l	#0xa4080014, r4
 3e28c:  9753	mov.w	#0x0fff, r7
 3e28e:  9253	mov.w	#0xd000, r2
 3e290:  e580	mov	#-128, r5
 3e292:  6141	mov.w	@r4, r1
 3e294:  2179	and	r7, r1
 3e296:  212b	or	r2, r1
 3e298:  2411	mov.w	r1, @r4
 3e29a:  d472	mov.l	<3e464>(#0xa40800d4), r4
 3e29c:  000b	rts
 3e29e:  2450	mov.b	r5, @r4