org 0x100
cpu 8086
bits 16

section .code

setup:
	; Save original video mode
	mov ah, 0xf
	int 0x10
	mov [original_video_mode], al

	; Move to VGA 320x200 256 color mode
	mov ah, 0
	mov al, 0x13
	int 0x10

	; Set es to point to VGA framebuffer
	mov ax, 0xa000
	mov es, ax

mainloop:
	; Check keyboard
	mov ah, 6
	mov dl, 0xff
	int 0x21
	jz .no_key

	.key:
		cmp al, 'q'
		je exit
		cmp al, 27
		je exit

	.no_key:
	; Check time
	mov ah, 0x2c
	int 0x21
	cmp ch, [hour]
	jne .changed
	cmp cl, [minute]
	jne .changed
	cmp dh, [second]
	jne .changed
	jmp .unchanged

	.changed:
		call clear_screen

		mov [hour], ch
		mov [minute], cl
		mov [second], dh

		mov ax, [second]
		mul word [six]
		mov bx, ax
		mov ax, [width]
		shr ax, 1
		shr ax, 1
		call sinx

		mov bp, [width]
		shr bp, 1
		mov bp, ax
		mov byte [es:bp], 7

	.unchanged:
	hlt
	jmp mainloop

exit:
	; Restore video mode
	mov ah, 0
	mov al, [original_video_mode]
	int 0x10

	ret

clear_screen:
	push bp

	xor bp, bp
	.loop:
		mov word [es:bp], 0
		add bp, 2
		cmp bp, 320*200
		jne .loop

	pop bp
	ret

; in:
;  ax = scale
;  bx = angle
; out:
;  ax = scale·cos(angle)
cosx:
	push bx

	; cos(x) = sin(x + 90°)
	add bx, 90
	call sinx

	pop bx
	ret

; in:
;  ax = scale
;  bx = angle
; out:
;  ax = scale·sin(angle)
sinx:
	push bx
	push dx

	.get_in_range:
		cmp bx, 360
		jb .in_range

		sub bx, 360
		jmp .get_in_range

		.in_range:

	; sin(90°) = 1
	.angle_90:
		cmp bx, 90
		je .end

	; Main branch of function 0 ≤ bx < 90
	.q1:
		cmp bx, 90
		ja .q2

		; Load sin(bx) * 2¹⁶ into bx
		shl bx, 1
		mov bx, [sin_table + bx]
		; Compute ax·bx and take high 16 bits, which cancel out the
		; pre-multiplication of the sin_table
		mul bx
		mov ax, dx

		jmp .end

	; sin(bx) = sin(180° - bx) for 90 < bx ≤ 180
	.q2:
		cmp bx, 180
		ja .q3_q4

		mov dx, 180
		sub dx, bx
		mov bx, dx
		call sinx

		jmp .end

	; sin(bx) = -sin(bx - 180°) for 180 < bx < 360
	.q3_q4:
		sub bx, 180
		call sinx
		neg ax

	.end:
	pop dx
	pop bx
	ret

section .rodata
width dw 320
height dw 200

sin_table:
	dw 0 ; sin(0°)
	dw 1143 ; sin(1°)
	dw 2287 ; sin(2°)
	dw 3429 ; sin(3°)
	dw 4571 ; sin(4°)
	dw 5711 ; sin(5°)
	dw 6850 ; sin(6°)
	dw 7986 ; sin(7°)
	dw 9120 ; sin(8°)
	dw 10252 ; sin(9°)
	dw 11380 ; sin(10°)
	dw 12504 ; sin(11°)
	dw 13625 ; sin(12°)
	dw 14742 ; sin(13°)
	dw 15854 ; sin(14°)
	dw 16961 ; sin(15°)
	dw 18064 ; sin(16°)
	dw 19160 ; sin(17°)
	dw 20251 ; sin(18°)
	dw 21336 ; sin(19°)
	dw 22414 ; sin(20°)
	dw 23486 ; sin(21°)
	dw 24550 ; sin(22°)
	dw 25606 ; sin(23°)
	dw 26655 ; sin(24°)
	dw 27696 ; sin(25°)
	dw 28729 ; sin(26°)
	dw 29752 ; sin(27°)
	dw 30767 ; sin(28°)
	dw 31772 ; sin(29°)
	dw 32767 ; sin(30°)
	dw 33753 ; sin(31°)
	dw 34728 ; sin(32°)
	dw 35693 ; sin(33°)
	dw 36647 ; sin(34°)
	dw 37589 ; sin(35°)
	dw 38521 ; sin(36°)
	dw 39440 ; sin(37°)
	dw 40347 ; sin(38°)
	dw 41243 ; sin(39°)
	dw 42125 ; sin(40°)
	dw 42995 ; sin(41°)
	dw 43852 ; sin(42°)
	dw 44695 ; sin(43°)
	dw 45525 ; sin(44°)
	dw 46340 ; sin(45°)
	dw 47142 ; sin(46°)
	dw 47929 ; sin(47°)
	dw 48702 ; sin(48°)
	dw 49460 ; sin(49°)
	dw 50203 ; sin(50°)
	dw 50931 ; sin(51°)
	dw 51643 ; sin(52°)
	dw 52339 ; sin(53°)
	dw 53019 ; sin(54°)
	dw 53683 ; sin(55°)
	dw 54331 ; sin(56°)
	dw 54963 ; sin(57°)
	dw 55577 ; sin(58°)
	dw 56175 ; sin(59°)
	dw 56755 ; sin(60°)
	dw 57319 ; sin(61°)
	dw 57864 ; sin(62°)
	dw 58393 ; sin(63°)
	dw 58903 ; sin(64°)
	dw 59395 ; sin(65°)
	dw 59870 ; sin(66°)
	dw 60326 ; sin(67°)
	dw 60763 ; sin(68°)
	dw 61183 ; sin(69°)
	dw 61583 ; sin(70°)
	dw 61965 ; sin(71°)
	dw 62328 ; sin(72°)
	dw 62672 ; sin(73°)
	dw 62997 ; sin(74°)
	dw 63302 ; sin(75°)
	dw 63589 ; sin(76°)
	dw 63856 ; sin(77°)
	dw 64103 ; sin(78°)
	dw 64331 ; sin(79°)
	dw 64540 ; sin(80°)
	dw 64729 ; sin(81°)
	dw 64898 ; sin(82°)
	dw 65047 ; sin(83°)
	dw 65176 ; sin(84°)
	dw 65286 ; sin(85°)
	dw 65376 ; sin(86°)
	dw 65446 ; sin(87°)
	dw 65496 ; sin(88°)
	dw 65526 ; sin(89°)

six dw 6

section .data
hour dw 0
minute dw 0
second dw 0

original_video_mode db 0