1) Which instructions to include:

• How many?
• Complexity - simple "ADD R1, R2, R3"

  complex e.g., VAX

  "MATCHC substrLength, substr, strLength, str"

2) Which built-in data types: integer, floating point, character, etc.

3) Instruction format:

• Length (fixed, variable)
• number of address (2, 3, etc)
• field sizes

4) Number of registers

5) Addressing modes supported - how are the memory addresses of variables/data determining

Number of Operands

3 Address	2 Address	1 Address (Accumulator machine)	0 Address (Stack machine)
MOVE (X Y)	MOVE (X Y)	LOAD M	PUSH M
		STORE M	POP M
ADD (X Y + Z)	ADD (X X + Y)	ADD M	ADD
SUB (X Y - Z)	ADD (X X - Y)	SUB M	SUB
MUL (X Y * Z)	MUL (X X * Y)	MUL M	MUL
DIV (X Y / Z)	DIV (X X / Y)	DIV M	DIV

D = A + B * C

3 Address	2 Address	1 Address (Accumulator machine)	0 Address (Stack machine)
MUL D, B, C ADD D, D, A	MOVE D, B MUL D, C ADD D, A	LOAD B MUL C ADD A STORE D	PUSH B PUSH C MUL PUSH A ADD POP D

Load/Store Architecture - operands for arithmetic operations must be from/to registers

LOAD R1, B

LOAD R2, C

MUL R3, R1, R2

LOAD R4, A

ADD R3, R4, R3

STORE R3, D

Flow of Control

How do we "jump around" in the code to execute high-level language statements such as if-then-else, while-loops, for-loops, etc.

Conditional branch - used to jump to "else" if x >= y

Unconditional branch - used to always jump "end if"

Labels are used to name spots in the code (memory)

("if:", "else:", and "end_if:" in below example)

Test-and-Jump version of the if-then-else (Used in MIPS)

if:

bge x, y, else

. . .

j end_if

else:

. . .

end_if:

Set-Then-Jump version of the if-then-else (Used in Pentium)

if:

cmp x, y

jge else

. . .

j end_if

else:

. . .

end_if:

The "cmp" instruction performs x - y with the result used to set the condition codes

SF - (Sign Flag) set if result is < 0

ZF - (Zero Flag) set if result = 0

CF - (Carry Flag) set if unsigned overflow

OF - (Overflow Flag) set if signed overflow

For example, the "jge" instruction checks to see if ZF = 1 or SF = 1, i.e., if the result of x - y is zero or negative.

Machine-Language Representation of Branch/Jump Instructions

a) direct/absolute addressing - the memory address of where the label resides is put into the machine language instruction (EA, effective address = direct)

e.g., assume label "end_if" is at address 8000₁₆

How relocatable is the code in memory if direct addressing is used?

How many bits are needed to represent a direct address?

b) Relative/PC-relative - base-register addressing where the PC is the implicitly referenced register