Addressing Modes, Branches and Loops

• 6502 addressing modes
• 6800 addressing modes
• 8085 addressing modes
• x86 addressing modes
• unconditional branches
• conditional branches
• compare and test
• test and set
• 6502 example
• 6800 example
• 8085 example
• x86 example

6502 Addressing Modes

• relative addressing only available for branches
• indirect addressing only available for JMP instruction: JMP has absolute and indirect modes
• in-class exercise: what is the indirect jump useful for?

More 6502 Addressing Modes

• zero page, X (Y): 8-bit offset added to 8-bit X (Y) register, truncated to 8 bits
• absolute, X (Y): 16-bit offset added to 8-bit X (Y) register
• indirect indexed LDA (aa), Y: aa and aa+1 contain a 16-bit pointer which is added to Y to give the effective address
• indexed indirect LDA (aa,X): aa+X and aa+X+1 contain a 16-bit pointer to the effective address
• in-class exercise: find a use for indirect indexed and a use for indexed indirect

6800 and 8085 Addressing Modes

• 6800 relative addressing: jumps and subroutine calls
• 6800 indexed addressing LDAA aa, X: the effective address is the 16-bit sum aa + X (index registers on 6800 are 16 bits)
• 8085 register indirect: 16-bit register pair, usually HL, holds the effective address

x86 Addressing Modes

• register indirect addressing MOV AL, [BX]: registers BP (uses stack segment), BX, SI, DI (data segment) EA is the contents of the register (plus segment)
• register relative addressing MOV AL, [BX + 89AB]: EA is the 16- (32-) bit sum of the contents of BX and the constant offset (plus segment)
• program relative addressing: JMP 333 at location 300, when compiled to machine code actually encodes the offset, 31
• program indirect addressing: JMP [BX+100] jumps to the address found at memory location pointed to by (BX + 100)
• base plus index addressing: MOV AX, [BX+DI]
• base relative plus index addressing: MOV AX, [BX+SI+10]

Unconditional Branches

• generally, jumps are absolute and branches are relative
• some architectures have a "skip" instruction which (conditionally) skips over the next instruction (often but not always a jump). For example, the PIC processors in Project 8 has skip instructions.
• unconditional branches are used:
  • to skip over else clause,
  • at end of while statements, and
  • (depending on the language) at the end of for statements

Conditional Branches

• jump if any one of the flag bits is (or is not) set
• different mnemonics: BEQ (Z flag is set), JNLE/JG (sign flag is clear),
• used for if, while, do, for statements
• different kinds of for statements:
  • C and Java: for is a specialized while statement
  • Pascal and others: count up or down a given number of times (but what if you change the loop variable within the loop body?)
• for specific count, usually easiest to count down to zero

Compare, Test and Set

• Compare: like subtract, but only changes the flags not the registers
• Test and Set: load a value from memory, "at the same time" setting it or one of its bits
• after the updated value is back in memory, we can check whether it was already set
• variants: load and increment, load and decrement
• useful for synchronization, as long as the operation is atomic
• processor does not release the memory bus during the operation

6502 Example

start:  LDX #00
        LDA #00
loop:   INX
        CMP $03A0, X
        BNE loop
done:   DEX
        STX $03A0
        BRK

6800 Example

start:  LDX #0000
        STX $0180
        LDX #0064
        STX $0182
loop:   LDX $0180
        LDAA $01A0,X
	INX
        STX $0180
        LDX $0182
        STAA $029F,X
        DEX
        STX $0182
        BNE loop
done:   WAI

8080/8085 Example

start:  LXI SP, 2000H
        MVI C, 14H
        LXI HL, 1234H
loop1:  MOV A, M
        PUSH PSW
        INX H
        DCR C
        JNZ loop1
        MVI C, 14H
        LXI HL, 1234H
loop2:  POP PSW
	MOV M, A
        INX H
        DCR C
        JNZ loop2

x86 Example

start:  CLD
        MOV SI, A000H
        MOV DI, B000H
        MOV CX, 0010H
        REP LODSB