A side: Assembly, the first language of the machine

Assembly speaks directly to the processor. The processor is the part of a computer that does the work. Each Assembly word is the name of one step that the processor can do.

One language, many dialects

Each processor family has its own Assembly. These are dialects of one idea, as British English and American English are dialects of English.

Dialect	Where it runs	Size of the dictionary
Intel 8086 (our choice)	IBM PC computers of the 1980s, and DOS	81 instructions
MOS 6502 family	Apple II, Commodore 64 and NES (close variants of the 6502)	56 instructions
RISC-V (base set)	New open-design chips	about 40 instructions
x86-64	Most desktop computers today	more than 1,000 instruction names

We learn the 8086 dialect because the game of this project runs on a copy of a 1980s PC. That copy runs inside your web browser. The 8086 dialect is small, old and well recorded. Many books and examples exist for it.

The parts of speech on the A side

• Verbs are instructions. MOV copies a value. ADD adds. JMP goes to a different place in the program.
• Nouns are registers, flags, ports and places in memory. A register is a small, fast box inside the processor. The 8086 has 14 register names, for example AX, BX and CX.
• Adjectives are size words. BYTE means 8 bits. WORD means 16 bits.
• Adverbs change how a verb acts. REP means “do the next verb again, CX times”.
• Idioms are ready-made services of the BIOS and DOS. The BIOS is a small program inside the computer. INT 21h with AH=09h means “show this line of text”.
• Grammar marks join the words. A comma separates the two operands. Square brackets mean “the value at this address”.
• Notes to the translator are directives. The assembler reads them, but the processor never sees them. ORG 100h tells the assembler where the program starts.

The shape of one sentence

Each line of Assembly is one sentence. It has up to four parts:

again:   add ax, bx    ; add BX to AX

Part	In the example	Meaning
Label	again:	A name for this place in the program
Verb	add	The action
Operands	ax, bx	The nouns: the destination first, then the source
Comment	; add BX to AX	A note for people. The assembler ignores it.

What the A side says well

• Each word is one exact step. You know the cost of every step.
• You can talk to the hardware directly: the screen memory, the keyboard chip and the clock chip.
• Programs are very small and very fast.

What the A side cannot say easily

• A formula needs many words.
• Each dialect is different. 8086 words do not work on a different processor.
• The language does not stop mistakes. A wrong address can stop the program.

The segments

Each segment below has an A page. Open a segment to see its strengths and its words.

• Keeping values: How do you tell the machine to remember a number?
• Doing arithmetic: How do you tell the machine to calculate?
• Making choices and repeating: How do you say “if” and “again”?
• Working with many values: How do you do one thing to a long list of values?
• Organizing a program: How do you divide a long text into parts with names?
• Showing text and reading keys: How does the program talk to the person at the keyboard?
• Controlling the hardware: How do you talk to the screen, the keyboard chip and the clock directly?