• Immediate addressing
• Direct addressing
• Register-indirect addressing
• Indexed addressing
• Implicit addressing
• Bit addressing
Addressing mode = Way of specifying operand
1. An instruction consists of
o Opcode (e.g., ADD, MOV,...) ~function
o Operand(s) (e.g., R3, #23H,...) ~ parameter
2. Where is the (value of) operand located?
o part of the instruction (immediate)
o in data memory (direct, indirect, indexed)
o in register; also, in a bit of a register/pin
Review: Register
1. 8-bit registers
o General purpose: R0, R1,... R7
o Special function: A, B, PSW, SP, I/O ports...
2. 16-bit registers
o DPTR (=DPH, DPL)
o PC is not an addressable register though
3. However! "Register addressing mode" refers to General Purpose one ONLY (R0..R7)
"Assembly" vs "Machine code" addressing mode
1. Book often assumes assembly-level
2. Strictly speaking, should be machine code representation of operand
3. Example: "register addressing mode"
o Machine level: R0...R7 only
o Assembly level: incl. all 8-bit reg, 16-bit
=>actually those are "direct mode"
Register addressing mode 8051
• Encoded as part of the instruction byte
• A is implicitly addressed; Rxxx is explicit
Immediate Addressing
• Immediate comes from "data value immediately follows the opcode byte"
o Meaning: constant value in an instruction
o Example:
Green part: immediate
Blue part : register
Immediate may be multiple bytes
1. Example: MOV imm to the 16-b data pointer
o MOV DPTR, #2550H ;; 2-byte immediate
o code is 3-bytes; 90 25 50
o DPTR is implicit
2. Assembler checks constant range
o MOV DPTR, #68975 ;; causes asm error
o #68975 is too large to fit in 2 bytes
Assembler label may be an immediate value
MOV DPTR, #Label
...
Label: DB "Hello world"
• The #Label part represents the address of the Label after the assembler determines its value
• fits the size of DPTR
Direct addressing
• Direct = address of operand
o on-chip memory
o Also mapped to general & special registers
o pointer in a separate byte, like immediates
• Usage: when you name a register not as R0...R7, but everything else that's addressable
o e.g., PSW, SP, I/O ports, DPH, DPL, address constant (as a label or constant)
Register vs Immediate vs direct addressing
• meaning of MOV A,0: take content at on-chip memory addr 0, copy it into the Accumulator
Subtle difference betw. Reg & Direct mode
1. MOV A, 0 ;;Direct mode => 2 bytes
o at on-chip address 0,
o mapped to R0 of bank 0 (four banks total)
2. MOV A, R0 ;;register mode =1 bytes!
o register R0 of current bank
o does not have to be bank zero!. Depends on PSW.3 and PSW.4
Special function registers
• Located at address >= 80H, up to FFH
o Also for Timer, Interrupt control, Serial port, Power control (not shown in table)
o Not all addresses are used
Implicit vs explicit direct addressing
• Accumulator has explicit address (E0H)
• Two ways: same functionality, but different encoding!
More example of implicit vs explicit:
• MOV DPTR, #2550H ;;3bytes
o This is implicit addressing for DPTR (DPTR does not have a direct address!)
• MOV DPL, #50H ;;3bytes
MOV DPH, #25H ;;3bytes
o DPL, DPH are explicitly direct addressing => each requires 1 bytes
o #50H, #25H each requires 1 byte
Implicit VS direct mode example cont'd
Limited combinations of addressing modes
• The opcode dictates operand's addr. modes
o Assembly mnemonic may look the same, but they may be different opcodes
Disallowed addr. mode combinations
• Register-to-register MOV
o e.g., MOV R1, R2
o solution: go through A or use immediate
• Accumulator to Accumulator MOV (useless)
• anything-to-immediate MOV (nonsense)
o e.g., MOV #20, R3
Addressing mode of PUSH/POP: direct mode
• Syntax:
o PUSH dir ;; push val at dir to stack
POP dir;; pop value to mem at dir
o Explicit: dir ;; on-chip mem location
o Implicit: SP (stack pointer)
• Example
o PUSH 05 ;; on-chip addr = 05
Cannot say PUSH R5 ;; register mode
Restrictions and Workarounds
• COL 3 : PUSH, POP are available only in direct mode
• Another idiosyncrasy of assembler: it could just translate
A =>0E 0H,
B =>0F 0H
Direct addressing VS indirect addressing
• Direct addressing (e.g, MOV A, 02)
o Pointer to operand is part of instruction (i.e., constant pointer, single-byte)
o example here: pointer = 02, 0n-chip memory
• ( Register ) Indirect (e.g., MOV A, @R0>
o Pointer to the operand value is found inside R0 or R1 (i.e., variable pointer)
o example here: pointer is found in R0
Register addressing vs Register-indirect addr
• Register addressing (e.g., MOV A, R1)
o R1 contains the operand value
• Register-indirect addressing (MOV A, @R1)
o R1 contains the pointer to operand value.
e.g., if R1 contains 20H, then operand is at on-chip memory address 20H
• R2,...R7 cannot be used with @
Example: Register vs Indirect mode
Copying constant to array: (1) direct
Copying constant to array: (2) indirect
Copying constant to array: (3) indirect + loop
Example: clear 16-byte array at address 60H
Example : loop 10 bytes *p++ = *q++;
Limitations of reg. indir. addressing in 8051
• For on-chip RAM only (1-byte pointer)
o 30H-7FH, SFR namely 80H -FFH
o Limited to R0 and R1 as pointers
• External RAM or on-chip ROM
o need to use DPTR (2-byte pointer)
Indexed Addressing Mode
• Index: array access
o Base address is in DPTR
o Index (offset) is in A
• Instruction in the form ;; regs are all implicit
o MOVC A, @A + DPTR
o C means "code memory"
• Meaning: A=DPTR [A];
Example LooKup Table: x-ssquared
• INT TAble []={0, 1, 4, 9, 16, 25, 36, 49, 64, 81};
for (;;) {P2 = Table [P1];}
• Assembly: Statically initialized array is DB:
Table: DB 0, 1, 4, 9, 16, 25, 36, 49, 64, 81
• Use DPTR for base pointer to Table:
MOV DPTR, #Table
• Lookup is
MOV A, P1 ;;P1 as index
MOVC A,@+DPTR ;;base +index
Program: write P2 = P1**2 continuosly
MOVC vs MOVX
• Both use 16-bit pointers
• MOVC: C= "code" space
• MOVX: X="external" data space
• Harvard architecture:
o separate code/data. e.g., 8051
• "Princeton" architecture:
o integrated code/data memory
Bit Addressing Mode
• Ability to specify a bit
o Normally, address are for bytes
• Bit addressable areas
o on-chip RAM at (byte-address) 20H--2FH each bit has a bit-address!
• I/0 ports P0...P3: 80H, 90H, A0H, B0H
• SFR: TCON, SCON, IE, IP, PSW, A, B, ACC
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