Assembly Language - Part 1

Intro - BBC Micro Assembly

When I was a child (a long time ago) I used to program a BBC Micro (or "Beeb") circa 1982. A Beeb had two programming languages, BBC Basic and 6502 Assembly language because it had a 6502 CPU. Nearly all the programs from those days were typed in from a magazine like BBC Micro User, most of them were BBC Basic but some had an assembly section delimited by square brackets to speed up, here is an example taken from this screenshot

>10 oswrch=&FFEE
>20 DIM memory% 100
>30 FOR Z=0 TO 3 STEP 3
>40 [OPTZ
>50 .start LDA#ASC"!"
>60 LDX #40
>70 .loop jsr oswrch
>80 dex:BNE loop
>90 rts:] NEXT Z
>100 CALL start
>110 END

I couldn't tell you what the above program does, it was a long time ago. But I can tell you that a high level programmer takes for granted writing an expression such as a=b+c. Under the hood the expression is translated into assembly operations: an accumulator is loaded with a memory call to the address of b and the same accumulator accumulates (hence its name) the sum with a memory call to the address of c and then the result is stored in the memory address of a.

So it's about marshalling data from memory into a register, doing an op and then storing the result. You may think 'so what'? But sometimes, a discussion of C/C++ compilers descends into assembly language and it is nice to have an inkling of how it works. But better to turn now to Intel Assembly Language.

Intel - x86 and x64 Assembly

So I won't reproduce the contents of the following links but will keep some notes. First the links.

• De Pauw Uni - Assembly Tutorial - Registers
• Uni of Alaska Fairbanks - x86_64 NASM Assembly Quick Reference ("Cheat Sheet")
• Brown University - x64 Cheat Sheet
• Brigham Young Uni - 8086 Assembly Language

Registers

Section	16 bit Code	Desc	Long Desc	32-bit	64-bit
General Purpose Registers	AX	Accumulator	Main arithmetic register	EAX	RAX
	BX	Base	Generally used as a memory base or offset	EBX	RBX
	CX	Counter	Generally used as a counter for loops	ECX	RCX
	DX	Data	General 16-bit storage, division remainder	EDX	RDX
Offset Registers	IP	Instruction pointer	Current instruction offset
	SP	Stack pointer	Current stack offset	ESP	RSP
	BP	Base pointer	Base for referencing values stored on stack	EBP	RBP
	SI	Source index	General addressing, source offset in string ops	ESI	RSI
	DI	Destination index	General addressing, destination in string ops	EDI	RDI
Segment Registers	CS	Code segment	Segment to which IP refers
	SS	Stack segment	Segment to which SP refers
	DS	Data segment	General addressing, usually for program's data area
	ES	Extra segment	General addressing, destination segment in string ops

  Flags Register (Respectively bits 11,10,9,8,7,6,4,2,0)
    OF  Overflow flag  : Indicates a signed arithmetic overflow occurred
    DF  Direction flag : Controls incr. direction in string ops (0=inc, 1=dec)
    IF  Interrupt flag : Controls whether interrupts are enabled
    TF  Trap flag      : Controls debug interrupt generation after instructions
    SF  Sign flag      : Indicates a negative result or comparison
    ZF  Zero flag      : Indicates a zero result or an equal comparison
    AF  Auxiliary flag : Indicates adjustment is needed after BCD arithmetic
    PF  Parity flag    : Indicates an even number of 1 bits
    CF  Carry flag     : Indicates an arithmetic carry occurred

Why do registers matter? It would appear that it is possible to get a compiler to pass variables not via the stack but via registers for extra speed. See Intel's C/C++ Calling Conventions.