I’ve been having some fun writing a Game Boy game with a co-worker, nothing fancy, just for fun to learn more about the Game Boy and teaching my co-worker how to write games for it in assembly. One of the problems we have is that the tools we use to debug the code are a little difficult to use. Basically you have to run it in an emulator/debugger like No$GMB and then hit break points and review the state of the device. I thought it would be really cool to just write assertions directly into the assembly code and have it tell me when things are wrong. Basically like unit testing the Game Boy code.

So I wrote both a non-graphical emulator and a custom assembler for the Game Boy.


WebASM Demo

Below is a demo of the C code compiled for WebASM. You can test out it’s functionality. To know how to use it, check out the descriptions below the demo.

The Emulator

The emulator is being developed in C for maximum portability. I mainly wrote the emulator for debugging purposes. I wanted an easy way to test subroutines for the Game Boy without having to load up a big graphical debugger in order to do so. Also those debuggers often lack the things I would like where are mainly assertions to prove that values are what they should be at a given point in the code/memory. So this emulator is not a graphical emulator and I still have yet to do any of the timing based parts of the Game Boy to call it a complete emulator. What it is good at right now is being able to run the opcode instructions given to it and manipulate registers and memory.

The Assembler

The assembler is being developed in C for maximum portability as well. In order to be able to make assertions directly in the code, it was important for me to write my own Assembler. It currently does not have any support for macros, IF statements or any fancy math, but it does have the ability to assemble the z80 code and labels into code that the emulator can run and test against. All of the Game Boy instruction set is supported in the current build of the Assembler and subroutines (with dot labels) are supported as well in order to be able to test loops, jumps and all that sort of stuff easily. It also strips comments from the code as well.


Though you can get a print out of the value of registers or number of clock cycles that have passed when doing assertions, you may not want to kill the program with the wrong assert value. For this reason I’ve added the ability to print out any of the registers in 8-bit mode or 16-bit pair mode (including the stack pointer and program counter). You can also print out the number of clock cycles that have passed (great for performance testing). Below is an example of how to print out things.

ld hl, $2021  ; Load a value int HL
ld [hl], $09  ; Save the value 9 to memory at HL
print hl      ; Print what the current value of HL is
print h       ; Print the high byte of HL
print l       ; Print the low byte of HL
print [hl]    ; Print the value in memory pointed to by HL

Available Prints

Below is a list of all prints that you can do currently.

Keywords Description
print a Prints the value in the A register
print f Prints the value in the F register
print b Prints the value in the B register
print c Prints the value in the C register
print d Prints the value in the D register
print e Prints the value in the E register
print h Prints the value in the H register
print l Prints the value in the L register
print af Prints the 16-bit value in the AF register pair
print bc Prints the 16-bit value in the BC register pair
print de Prints the 16-bit value in the DE register pair
print hl Prints the 16-bit value in the HL register pair
print [bc] Prints the value in memory pointed to by [BC]
print [de] Prints the value in memory pointed to by [DE]
print [hl] Prints the value in memory pointed to by [HL]
print sp Prints the value of the stack pointer
print pc Prints the value of the program counter
print clocks Prints the number of clock cycles that have passed

Writing Assertions

Here is an example of what the Assembler can do with assertions.

; Subtract BC from HL and store the result in HL
	ld hl, $1104	; Our test LHS
	ld bc, $1005	; Our test RHS
	push af
	ld a, l			; Get low byte
	sub c			; Subtract rhs low byte
	jr nc, .skip	; If we didn't go negative, jump to skip
	dec h			; Otherwise we decrement high byte
	ld l, a			; Set low byte to new value
	ld a, h			; Get high byte
	sub b			; Subtract rhs high byte
	ld h, a			; Set high byte to new value
	pop af
	assert eq hl, $00FF

What you will see in the code above we have a line assert eq hl, $00FF. This will test the code immediately after pop af has ran to determine if the value in HL is euqal to the value $00FF. This will then print out to the console if the assertian has passed or failed. This allows for quickly testing out subroutines to make sure they work as expected.

Since the assertions are available on the state of the machine, you can use assertions to dynamically check code while it is running instead of doing static checking on that specific line. For example, here is some code that uses the e register as a temporary value to use in the assert in each iteration through the loop.

	ld a, $09		; Increment value in memory at address $FF00 9x
	ld hl, $FF00		; Address to increment
	ld e, $00		; Our assertion checking device
	ld [hl], e		; Start our value off as 0
	inc [hl]		; Increment the value at $FF00
	inc e			; Increment our sanity checker
	dec a			; Decrement our loop counter
	assert eq [hl], e	; Assert on our dynamic value
	jr nz, .loop

The above code uses the e register as a temp value to use in the assertions. Of course this follows the same rules for non-asserted code, so you’d probably want to push whatever is inside of e to save and restore it if you are going to do something like this.

Also, for those of you who enjoy counting clock cycles to see how fast you can make a piece of code, you can assert on cycles as well. You can check ==, !=, <=, >=, <, > in your clocks assert. Below is an example of checking clock cycles at a given line. You can imagine checking eq clocks might not be as useful as using lt though!

; Just loading up some stuff, both take 3 clock cycles
ld hl, $1104	; Our test LHS
ld bc, $1005	; Our test RHS
assert eq clocks, $06

Available Assertions

Below are 3 tables, the first table is explaining the syntax used, the second is the comparison options, and the third are the actual assertions (reference the 2 tables above it).


Keyword Description
R Any 8-bit register (a, f, b, c, d, e, h, l)
RR Any 16-bit register pair (af, bc, de, hl)
%x Any 8-bit number (5, $3A)
%xx Any 16-bit number (536, $3A9E)
%xxxx Any 32-bit number (12345678, $F36B3A9E)
<=> Comparison operator (eq, neq, leq, geq, lt, gt)
clocks The number of clock cycles that have passed since start

Comparison operators

Keyword Description
eq Are equal
neq Are not equal
leq Left is less than or equal to right
geq Left is greater than or equal to right
lt Left is less than right
gt Left is greater than right

Assertion instructions

Format Example Description
assert <=> R, %x assert eq a, $3F Compares a register to an 8-bit value
assert <=> R, R assert neq b, e Compares the value of 2 registers
assert <=> RR, RR assert leq bc, de Compares the values of 2 16-bit register pairs
assert <=> RR, %xx assert geq de, $020F Compares the values of a 16-bit register to a 16-bit value
assert <=> [RR], %x assert lt [hl], $03 Compares the value in memory at address held in 16-bit register pair to an 8-bit value
assert <=> [RR], R assert gt [hl], e Compares the value in memory at address held in 16-bit register pair to a register value
assert <=> [%xx], %x assert eq [$3F9A], $09 Compares the value in memory at address to an 8-bit value
assert <=> [%xx], R assert eq [$2000], a Compares the value in memory at address to a register value
assert <=> clocks, %xxxx assert lt clocks, 9 Compares the number of clock cycles that have passed to the given value

Game Boy OpCodes

Instruction OpCode Clocks
nop 00 1
ld bc, %xx 01 3
ld [bc], a 02 2
inc bc 03 2
inc b 04 1
dec b 05 1
ld b, %x 06 2
rlca 07 1
ld [%xx], sp 08 5
add hl, bc 09 2
ld a, [bc] 0A 2
inc bc 0B 2
inc c 0C 1
dec c 0D 1
ld c, %x 0E 2
rrca 0F 1
ld de, %xx 11 3
ld [de], a 12 2
inc de 13 2
inc d 14 1
dec d 15 1
ld d, %x 16 2
rla 17 1
jr %x 18 2
add hl, de 19 2
ld a, [de] 1A 2
inc de 1B 2
inc e 1C 1
dec e 1D 1
ld e, %x 1E 2
rra 1F 1
jr nz, %x 20 2
ld hl, %xx 21 3
ld [hli], a 22 2
inc hl 23 2
inc h 24 1
dec h 25 1
ld h, %x 26 2
daa 27 1
jr z, %xx 28 2
add hl, hl 29 2
ld a, [hli] 2A 2
inc hl 2B 2
inc l 2C 1
dec l 2D 1
ld l, %x 2E 2
cpl 2F 1
jr nc, %x 30 2
ld sp, %xx 31 3
ld [hld], a 32 2
inc sp 33 2
inc [hl] 34 3
dec [hl] 35 3
ld [hl], %x 36 3
scf 37 1
jr c, %x 38 2
add hl, sp 39 2
ld a, [hld] 3A 2
inc sp 3B 2
inc a 3C 1
dec a 3D 1
ld a, %x 3E 2
ccf 3F 1
ld b, b 40 1
ld b, c 41 1
ld b, d 42 1
ld b, e 43 1
ld b, h 44 1
ld b, l 45 1
ld b, [hl] 46 2
ld b, a 47 1
ld c, b 48 1
ld c, c 49 1
ld c, d 4A 1
ld c, e 4B 1
ld c, h 4C 1
ld c, l 4D 1
ld c, [hl] 4E 2
ld c, a 4F 1
ld d, b 50 1
ld d, c 51 1
ld d, d 52 1
ld d, e 53 1
ld d, h 54 1
ld d, l 55 1
ld d, [hl] 56 2
ld d, a 57 1
ld e, b 58 1
ld e, c 59 1
ld e, d 5A 1
ld e, e 5B 1
ld e, h 5C 1
ld e, l 5D 1
ld e, [hl] 5E 2
ld e, a 5F 1
ld h, b 60 1
ld h, c 61 1
ld h, d 62 1
ld h, e 63 1
ld h, h 64 1
ld h, l 65 1
ld h, [hl] 66 2
ld h, a 67 1
ld l, b 68 1
ld l, c 69 1
ld l, d 6A 1
ld l, e 6B 1
ld l, h 6C 1
ld l, l 6D 1
ld l, [hl] 6E 2
ld l, a 6F 1
ld [hl], b 70 2
ld [hl], c 71 2
ld [hl], d 72 2
ld [hl], e 73 2
ld [hl], h 74 2
ld [hl], l 75 2
halt 76 1
ld [hl], a 77 2
ld a, b 78 1
ld a, c 79 1
ld a, d 7A 1
ld a, e 7B 1
ld a, h 7C 1
ld a, l 7D 1
ld a, [hl] 7E 2
ld a, a 7F 1
add b 80 1
add c 81 1
add d 82 1
add e 83 1
add h 84 1
add l 85 1
add [hl] 86 2
add a 87 1
adc b 88 1
adc c 89 1
adc d 8A 1
adc e 8B 1
adc h 8C 1
adc l 8D 1
adc [hl] 8E 2
adc a 8F 1
sub b 90 1
sub c 91 1
sub d 92 1
sub e 93 1
sub h 94 1
sub l 95 1
sub [hl] 96 2
sub a 97 1
sbc b 98 1
sbc c 99 1
sbc d 9A 1
sbc e 9B 1
sbc h 9C 1
sbc l 9D 1
sbc [hl] 9E 2
sbc a 9F 1
and b A0 1
and c A1 1
and d A2 1
and e A3 1
and h A4 1
and l A5 1
and [hl] A6 2
and a A7 1
xor b A8 1
xor c A9 1
xor d AA 1
xor e AB 1
xor h AC 1
xor l AD 1
xor [hl] AE 2
xor a AF 1
or b B0 1
or c B1 1
or d B2 1
or e B3 1
or h B4 1
or l B5 1
or [hl] B6 2
or a B7 1
cp b B8 1
cp c B9 1
cp d BA 1
cp e BB 1
cp h BC 1
cp l BD 1
cp [hl] BE 2
cp a BF 1
ret nz C0 2
pop bc C1 3
jp nz, %xx C2 3
jp %xx C3 3
call nz, %xx C4 3
push bc C5 4
add a, %x C6 2
rst 00H C7 8
ret z C8 2
ret C9 2
jp z, %xx CA 3
call z, %xx CC 3
call %xx CD 3
adc a, %x CE 2
rst 08H CF 8
ret nc D0 2
pop de D1 3
jp nc, %xx D2 3
call nc, %xx D4 0
push de D5 0
sub a, %x D6 2
rst 10H D7 8
ret c D8 2
reti D9 2
jp c, %xx DA 3
call c, %xx DC 3
sbc a, %x DE 0
rst 18H DF 8
ld [$ff00+c], a E0 3
pop hl E1 3
ld [c], a E2 2
push hl E5 4
and %x E6 2
rst 20H E7 8
add sp, %x E8 4
jp [hl] E9 1
ld [%xx], a EA 4
xor %x EE 2
rst 28H EF 8
ld a, [$ff00+c] F0 3
pop af F1 3
ld a, [c] F2 2
di F3 1
push af F5 4
or %x F6 2
rst 30H F7 8
ldhl sp, %x F8 3
ld sp, hl F9 2
ld a, [%xx] FA 4
ei FB 1
cp %x FE 2
rst 38H FF 8
swap a CB37 2
swap b CB30 2
swap c CB31 2
swap d CB32 2
swap e CB33 2
swap h CB34 2
swap l CB35 2
swap [hl] CB36 4
rlc a CB07 2
rlc b CB00 2
rlc c CB01 2
rlc d CB02 2
rlc e CB03 2
rlc h CB04 2
rlc l CB05 2
rlc [hl] CB06 4
rl a CB17 2
rl b CB10 2
rl c CB11 2
rl d CB12 2
rl e CB13 2
rl h CB14 2
rl l CB15 2
rl [hl] CB16 4
rrc a CB0F 2
rrc b CB08 2
rrc c CB09 2
rrc d CB0A 2
rrc e CB0B 2
rrc h CB0C 2
rrc l CB0D 2
rrc [hl] CB0E 4
rr a CB1F 2
rr b CB18 2
rr c CB19 2
rr d CB1A 2
rr e CB1B 2
rr h CB1C 2
rr l CB1D 2
rr [hl] CB1E 4
sla a CB27 2
sla b CB20 2
sla c CB21 2
sla d CB22 2
sla e CB23 2
sla h CB24 2
sla l CB25 2
sla [hl] CB26 4
sra a CB2F 2
sra b CB28 2
sra c CB29 2
sra d CB2A 2
sra e CB2B 2
sra h CB2C 2
sra l CB2D 2
sra [hl] CB2E 4
srl a CB3F 2
srl b CB38 2
srl c CB39 2
srl d CB3A 2
srl e CB3B 2
srl h CB3C 2
srl l CB3D 2
srl [hl] CB3E 4
bit a CB47 2
bit b CB40 2
bit c CB41 2
bit d CB42 2
bit e CB43 2
bit h CB44 2
bit l CB45 2
bit [hl] CB46 4
set a CBC7 2
set b CBC0 2
set c CBC1 2
set d CBC2 2
set e CBC3 2
set h CBC4 2
set l CBC5 2
set [hl] CBC6 4
res a CB87 2
res b CB80 2
res c CB81 2
res d CB82 2
res e CB83 2
res h CB84 2
res l CB85 2
res [hl] CB86 4
stop 1000 1