Assembly Language

Computer language hierarchy

• High Level Language • Assembly language • Machine language • Compiled or interpreted • One statement maps to one or more assembly language statement • Assembled • One statement maps to one machine language instruction • Loaded directly into the processor memory • One statement maps to one or more RTL statements

Computer language hierarchy

• You can toss in the scripting languages (PERL, Python, PHP, VBScript…) • You can toss in the interpreted languages (BASIC, Haskell, Lisp, Forth) • And other categories • The seem to fit somewhere between HLL and Assembly but don’t run directly on the hardware (virtual machine/interpreter)

Assembly language

• This is as close as you ever get to the hardware (without actually keying in binary values – which no one ever does anymore) • Requires a major shift in mindset from programming HLLs

Programming

• In HLL programming you concern yourself with variables, datatypes, and control statements – int, float, char, if, for, while, arrays, etc.

• In assembly language programming you concern yourself with memory and instruction – RAM, ROM, registers, instruction classes, addressing modes

Programming

• In HLL programming you concern yourself with keywords and grammars – Statements, expressions, etc.

• In assembly language programming you concern yourself with mnemonics and circuit components – Instruction names, arithmetic units, control units, etc.

Programming

• In HLL programming you sit down with a “how to” textbook to guide you • In assembly language programming you sit down with a chip specification document to guide you

Programming

• In HLL programming you might be able to

estimate

how long a program will take to run and how much memory it will use • In assembly language programming you can

calculate

how long a program will take to run and how much memory it will use

Assembly programming

• You must have an understanding of the architecture on which you are running your program • Each architecture [or family] has it’s own assembly language – The language is intimately tied to the design of the architecture

The 8051 architecture

• • • • • • • • • • 8-bit CPU with A (accumulator) and B registers 16-bit program counter and data pointer 8-bit program status word 8-bit stack pointer Internal ROM Internal RAM – Four register banks of 8 registers each – Sixteen bytes of bit addressable locations – Eighty bytes of general purpose data memory – Thirty two input/output pins Two 16-bit timer/counters Various control registers Serial port Interrupt sources

The 8051 instruction set

• Instruction classes – Arithmetic operations – Logical operations – Data transfer – Boolean variable manipulation – Program branching

The 8051 instruction set

• The processor status word (PSW) – Carry flag – Auxiliary carry flag – General purpose flags (2) – Register bank selector (2 bits) – Overflow flag – Parity flag

The 8051 instruction set

• Addressing modes – Register – Direct – Indirect-register – Constant 8-bit – Constant 16-bit – Address 16-bit – Address 11-bit – Relative – Bit

Addressing modes

• Rn where 0 ≤ n ≤ 7 – the contents of register n; • 0x00 (or other hex number) – the contents of a memory location • @Ri where 0 ≤ i ≤ 1 – indirect, register holds the memory address • #0x00 (or other hex number) – 8-bit constant number • #0x00 16 (or other hex number) – 16-bit constant number • There are a few others but these are the most commonly used modes

Data transfer instructions

• MOV op-code • Various operands dependant on desired addressing mode – Some examples:

MOV A, R0 ; A = R0 MOV A, #0x00 ; A = 0 MOV A, 0x00 ; A = memory[0] MOV A, @R0 ; A = memory[R0] MOV R0, A ; R0 = A

Simulator

• We don’t have an 8051 processor • The next best thing is a simulator • Download EdSim51 from EdSim51 – This is a Java executable (.jar) file that runs on both Windows and MacOS • Documentation is available on the download site