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Assemlber Instruction format

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• Meaning of the Assembler Instructions (fetched from memory !)
  • The assembler encoding is designed by each CPU manufacturer to their own liking...

    Various motivations of differing importance will dictate the design, such as: fixed vs. variable length, how many registers does the CPU have, what can each register do, etc.

    We will use a very simple-minded assembler code to illutsrate the CPU. Also, a simple assembler code will simplify the decoding process.

  • The assembler code that our CPU will execute will be:
    • Fixed length
    • have very limited addressing modes

  • The following is the assembler instruction encoding that we will use:

    Instruction code	Mnemonic	Instruction	Meaning
    0000xxxxxxxxxxxx	LODD	Load Direct	ac <- mem[x]
    0001xxxxxxxxxxxx	STOD	Store Direct	mem[x] <- mem[x]
    0010xxxxxxxxxxxx	ADDD	Add Direct	ac <- ac + mem[x]
    0011xxxxxxxxxxxx	SUBD	Subtract Direct	ac <- ac - mem[x]
    0100xxxxxxxxxxxx	JPOS	Jump positive	if ac > 0 then pc <- x
    0101xxxxxxxxxxxx	JZER	Jump zero	if ac = 0 then pc <- x
    0110xxxxxxxxxxxx	JUMP	Jump always	pc <- x
    0111xxxxxxxxxxxx	LODC	Load constant	ac <- x
    1000xxxxxxxxxxxx	LODL	Load local	ac <- mem[sp + x]
    1001xxxxxxxxxxxx	STOL	Store local	mem[sp + x] <- ac
    1010xxxxxxxxxxxx	ADDL	Add local	ac = ac + mem[sp + x]
    1011xxxxxxxxxxxx	SUBL	Subtract local	ac = ac - mem[sp + x]
    1100xxxxxxxxxxxx	JNEG	Jump negative	if ac < 0 then pc <- x
    1100xxxxxxxxxxxx	JNEG	Jump negative	if ac < 0 then pc <- x
    1101xxxxxxxxxxxx	JNZE	Jump nonzero	if ac != 0 then pc <- x
    1110xxxxxxxxxxxx	CALL	Call subroutine	sp <- sp-1; mem[sp] <- pc; pc <- x
    1111000000000000	PUSHI	Push Indirect	sp <- sp-1; mem[sp] <- mem[ac]
    1111000100000000	POPI	Pop Indirect	mem[ac] <- mem[sp]; sp <- sp+1;
    1111001000000000	PUSH	Push on stack	sp <- sp-1; mem[sp] <- ac
    1111010000000000	POP	Pop off stack	ac <- mem[sp]; sp <- sp+1;
    1111100000000000	RET	Return from subroutine	pc <- mem[sp]; sp <- sp+1;
    1111101000000000	SWAP	Swap ac and sp	tmp <- ac; ac <- sp; sp <- tmp
    11111100yyyyyyyy	INCSP	Increment sp	sp <- sp + y
    11111110yyyyyyyy	DECSP	Decrement sp	sp <- sp - y