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10.15 ASM Language

1.1 Introducing the platform

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  • Nintendo Game Boy: videogame console
  • Released in 1989
  • 120 million units sold worldwide
  • Low cost, low energy consumption
  • 8 bits CPU (name: SM83, sometimes called GBz80)
  • 160x144 pixels display, 4 shades of green
  • Still relevant in 2023 for education, research and fun

1.2 Game examples

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1.3 The Game Boy CPU

A custom CPU made by Sharp (Japan)

It imitates closely two existing popular CPUs:

  • Intel 8080: used in personal computers, the precursor of the x86 family still in use today
  • Zilog Z80: used in many personal computers of the 1970s and 1980s, and pocket calculators

1.4 Our plan

  • first weeks: get to know the CPU
  • then: get to know the platform
  • a computer is not only the CPU itself!
  • it is: CPU + memory map + graphic device + buses

  • later weeks of semester: get the platform to work to run graphical programs

  • we will cover ~60% of the platform

  • objectives:

  • get experience in assembly programming

  • understand the implications of defining a stored program without a compiler
  • understand low-level issues of timing, instruction sizes

1.5 Instructions and instructions set

  • Instructions are used to control a CPU
  • Each instruction causes the CPU to perform a very specific task
  • Each CPU has its own instruction set, that is a fixed set of instructions that it can understand.
  • A program in machine code consists of a sequence of machine instructions and will only work on a specific CPU model

1.6 The CPU

                     CPU
                +------------+
                |            |
instructions -> |  internal  |
                |   state    |
                |            |
                +------------+
inside the CPU, there are some data

                     CPU
                +-------------+
                |   8-bit     |
instructions -> |  registers  |
                |             |
                |a,b,c,d,e,h,l|
                +-------------+
registers are vary small and call store 8-bit. Like memory, but very small and is in CPU

...               CPU
instr1           +------------+
instr2    ->     |   8-bit    |
instr3           |  registers |
...              |            |
                 |a,b,c,d,e,h,l|
                 +------------+

  • What are registers?

  • each register == 8 bits

  • think of them as C variables char/uint8
  • cannot add more registers
  • ​a is often called the "accumulator"

  • What are instructions?

  • small commands that tell the CPU to do something

  • very simple syntax

1.7 Assembly language (ASM)

  • ASM == writing programs as a sequence of instructions
  • almost no syntax!

  • instructions are very different from C statements

  • there are infinitely many C statements because we can extend the statements are large as we want, e.g.: a=b+c+d+e+f+g+...

  • there are finitely many ASM instructions

...
...

Example:

In C:

a = 20;
b = 40;
c = 50;
d = (a + b) + c;

In ASM:

ld a,20
ld b,40
ld c,50
add b
add c
ld d,a

​ld means load data

All arithmetic operations use a as the first data implicitly.

​a​ is accumulator, the result is temporarily stored in a

1.8 ASM syntax

ld a,10          ; decimal notation
ld b,20
add b             ; a = a + b
ld b,a
ld a,%00001111    ; binary notation
ld c,%11000000
or c              ; a = a | c
ld c,$F0          ; hexadecimal notation
xor c             ; a = a ^ c
Format type Prefix Accepted characters
Hexadecimal $ 0123456789ABCDEF
Decimal none 0123456789
Binary % 01
  • A numerical constant is also called an "immediate" in assembly.
  • The syntax is line-based, meaning that you do one instruction per line.
  • Uppercase/lowercase does not matter

1.9 Today's instructions

We will cover part of the following:

  • 8-bit load instructions
  • 8-bit arithmetic and logic instructions
  • arithmetic shift instructions

Then do a few exercises.

1.10 8-bit load instructions

Mnemonic Description
​ld r,r ​r=r
​ld r,n ​r=n
  • ​r​ is one of the registers a,b,c,d,e,h,l
  • ​n is a 1 byte constant

  • examples:

  • ​ld a,b

  • ​ld b,a
  • ​ld b,10
  • ​ld b,%0001010 (equivalent to previous one)
  • ​ld h,l

1.11 increment/decrement instructions

Mnemonic Description
inc r r=r+1
dec r r=r-1

  • examples:

  • ​inc a

  • ​inc b
  • ​inc c
  • ​dec d
  • ​dec e
  • ​dec h ...

image

LD A, \(\emptyset \leftarrow\) 00000000
DEC A \(\leftarrow\) 11111111

1.12 8-bit arithmetic instructions with register or constant arguments

Mnemonic Description
add r A=A+r
add n A=A+n
sub r A=A-r
sub n A=A-n
  • ​a is always the first argument of these instructions
  • ​a is also called the "accumulator"

  • in some documentation, they are written with a as first argument:

  • ​add a,b

  • ​sub a,c the essence of it is plus the negative one
  • the Game Boy CPU has no multiply and divide instructions

1.13 8-bit logic instructions with register or constant arguments

Mnemonic Description
and r A=A & r
and n A=A & n
xor r A=A ^ r
xor n A=A ^ n
or r A=A | r
or n A=A | n
cpl A = A xor FF(=\(11111111_2\)) (invert all bits of A)

image

1.14 Arithmetic Shift instructions

Mnemonic Description
sla r shift left arithmetic (b0=0)
sra r shift right arithmetic (b7=b7)

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If this register has an unsigned number, use SRL; otherwise, use SRA.

UNSIGNED CHAR B = 151;
B = B / 2;

After compiling,
..... SRL B.....

UNSIGNED CHAR B = -105;
B = B / 2;

After compiling,
..... SRA B.....

1.15 Exercise 1

What are the final values of all registers of this snippet?

LD A,0
LD B,0
INC A
INC B
ADD B
LD C,A
LD D,10
ADD D

You can write the answer in the most convenient way (decimal, binary or hexa).
When writing answer in decimal, use the most convenient way between unsigned and signed.
E.g., -128 and 128 are considered the same number; -1 and 255 are considered the same number.

Solution

A=12, B=1, C=2, D=10

1.16 Exercise 2

What are the final values of all registers of this snippet?

LD A,100
LD B,50
LD C,20
SUB B
SUB C
SUB C
DEC B
INC C

Solution

A=10, B=49, C=21

1.17 Exercise 3

LD A,0
LD B,255
DEC A
INC B

Solution

A=255, B=0

LD A,100
LD B,100
ADD B
ADD B

Solution

A=44, B=100

LD A,100
LD B,100
SUB B
SUB B

Solution

A=156, B=100

1.18 Exercise 4

LD A,%00000000
LD B,%10101010
OR B

Solution

A=%10101010, B=%10101010

LD A,%00001111
LD B,%10101010
OR B

Solution

A=%10101111, B=%10101010

LD A,%00001111
LD B,%10101010
AND B

Solution

A=%00001010, B=%10101010

1.19 Exercise 5

LD A, $0F
LD B, $AA
XOR B

Solution

A=%10100101, B=%10101010

LD A, $0F
LD B, $BB
CPL
LD B,A

A=%11110000, B=%11110000