ASM Snippets

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Here are some useful and common ASM snippets. You may encounter them in others' code, or use them to solve situations yourself. It's a good idea to read all of the snippets and pick the best one for your use case, depending on the use.

Sign extension

Assuming we have a value, we may want to sign-extend it. It's possible using as little as two instructions!

 add a, a ; Shift MSB into carry
 sbc a, a

Bytes: 2; Cycles: 2

16-bit arithmetic

It's tempting to answer that all the work is already done by hl instructions, but unlike the z80, the GB CPU can only perform 16-bit additions. The rest must be implemented by hand as the following examples do:

 ld a, l
 sub a, {low byte}
 ld l, a
 ld a, h
 sbc a, {high byte}
 ld h, a
 ld a, l
 adc a, {low byte}
 ld l, a
 ld a, h
 adc a, {high byte}
 ld h, a

This pattern can also be used for additions that can't be carried out using `add hl, reg16`, such as adding a value to some memory address, or when hl is clobbered.

16-bit and 8-bit

Often, we find ourselves the need to add an 8-bit offset to a 16-bit position. Common examples include accessing fields in a struct, or adding an 8-bit vector to a 16-bit value. It's tempting to use the above pattern with the high byte as zero, but it's possible to do better:

 ; Assuming A already contains the value to be added...
   add a, l
   ld l, a
   jr nc, .noCarry
   inc h

This, however, requires creating a label, which is tedious in some cases (such as older assemblers without anonymous labels). There is however an alternative, slightly trickier to understand, perfectly equal in size and speed, that doesn't use any labels.

 add a, l ; a = low + old_l
 ld l, a  ; a = low + old_l = new_l
 adc a, h ; a = new_l + old_h + carry
 sub l    ; a = old_h + carry
 ld h, a

Multiplying a signed 8-bit number by an unsigned one

Multiply signed H by unsigned C. If H is negative, it works using the distributive property. We want (unsigned H - 256) * C, but this is the same as (Unsigned H * C) + (256 * -C). So if H is negative, we put -C into A (which is otherwise zero), and add it to the high byte of the product (thus multiplying it by 256)

    xor a ;clear our workspace
    ld l, a
    ld b, a 
    ; special first iteration
    add hl, hl ; this shifts the sign of the signed multiplier into carry
    jr nc, .positive
    ; for the multiplication part, load instead of add for this first round
    ld l, c
    ; get negative c into a to add it later
    sub c
    ; now a is negative c if h was negative, or zero if x was positive
    REPT 7 ; the other 7 iterations are standard binary long multiplication
    add hl, hl
    jr nc, :+
    add hl, bc
    ; finish off by adding a to h to correct for the sign
    add a, h
    ld h, a

Advancing to the next aligned struct

One of the largest issues when processing a struct is that we don't always find ourselves at a constant offset within the struct (for example, if you stop processing a NPC because it's actually far off-screen, you'll be in a different position than if you had read until the end. Thus, it's useful to be able to go to the beginning of the struct (or of the next one) at any point

Assuming we have a pointer to anywhere within a struct in any 16-bit register, advance to the next one. (Example given with the 16-bit register being de)

Requirements: The size of the struct must be a power of two.

Clobbers: A

 ld a, e
 or {size of struct}-1
 ld e, a
 inc de

Bytes: 5; Cycles: 6

Without page crossing

If the high byte of the pointer is constant across all structs, it's possible to save one cycle:

 ld a, e
 or {size of struct}-1
 inc a
 ld e, a

It also puts the low byte of the next struct's address in A, which is useful for termination comparisons:

 ld a, e
 or 16-1
 inc a
 ld e, a
 cp LOW(wArrayEnd)
 jr c, .processStruct


Set carry if two values match

Set the carry flag based on whether the value in A is zero. This can be useful to feed a following adc, sbc, rra, rr, or rl instruction.

 cp $01   ; Set carry if A == 0
          ; or
 add $FF  ; Set carry if A != 0
          ; or
 cp $01   ; Set carry if A != 0 and preserve A

Set the carry flag based on comparing A to another value.

 sub b
 cp $01   ; Set carry if A == B
          ; or
 sub b
 add $FF  ; Set carry if A != B

Less optimized snippets

These are common snippets which are less optimized than the ones shown above, but are documented regardless to aid at reading them. Note that the snippet you are looking for might slightly differ. Look at each one, and consider if a 16-bit register couldn't be swapped for another, for instance.

Adding a 8-bit number to a 16-bit one

In hl:

 ld c, a
 ld b, 0
 add hl, bc

In another register:

 ld l, a
 ld h, 0
 add hl, de
 ld d, h
 ld e, l

Those clobber hl and another registers = 4 out of 7. They do, however, preserve a and the Z flag.

Advancing to the next aligned struct

 ld a, l
 and -{size of struct}
 add a, {size of struct}
 ld l, a

This is a more naive implementation of the "advance to next struct" snippet, but it's bigger by 1 byte, and 1 cycle slower.

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