of the instruction to be accessed from the Program Memory.
Base-Register- plus Index-Register- Indirect Addressing simplifies accessing look-up-tables (LUT) resident in Program Memory. A byte may be accessed from a LUT via an indirect move from a location whose address is the sum of a base register (the DPTR or PC) and the index· register (A).
INTERNALSPECIAL
DATA RAMIFUNCTION
STACKREGISTERS
~~
255 | 255 | 248 | F8H | |
248 | | | FOH | |
240 | | | |
| | E8H | |
232 | | | |
| | EOH | |
224 | | | |
| | D8H | |
216 | | | |
| | DOH | |
208 | | | |
| | C8H | |
200 | | | DIRECT |
| | COH |
192 | | | ADDRESS- |
| | B8H |
164 | | | ING |
176 | | | BOH | (BITS) |
| | | |
168 | | | ASH | |
160 | | | AOH | |
152 | | | 98H | |
144 | | | 90H | |
136 | | | 88H | |
128 | 135 | 128 | 80H | |
| | | ..,.....:::.- |
DIRECT | ~ | 127 | 120 |
|
ADDRESSING | | | |
(BITS) | .E. | 7 | 0 |
|
| | R7 | BANK 3 |
| 24 | RO |
| |
| | R7 | BANK2 |
REGISTER | .!! | RO | |
ADDRESSING | , | R7 | BANK 1 |
| 8 | RO |
| |
| | R7 | BANKO |
| 0 | RO | |
| '- | | |
"~--~y~) DIRECT ADDRESSING
STACK-POINTER REGISTER-INDIRECT AND
REGISTER-INDIRECT ADDRESSING
Figure 2.16. Addressing Operands In
Internal Data Memory
2.5 DATA MANIPULATION
The 8051 microcomputer is efficient both as an arith- metic processor and as a controller. In addition to the capabilities of its 8048 predecessor, the 8051 was enhanced with improved data transfer, logic manipulation, arith- metic processing, and real-time control capabilities. The 8051 performs operations on bit, nibble (4-bit), byte (8-bit) and double-byte (l6-bit) data types. It is classified as an 8-bit machine since the internal ROM, RAM, Special Function Registers, Arithmetic/ Logic Unit (ALU) and the external data bus are each 8-bits wide. The double-byte data type is used only by the Data Pointer and the Program Counter. The Data Pointer can be manipulated as a single double-byte register (DPTR) or
as two locations in Internal Data Memory (DPH & DPL). The Program Counter is always manipulated as a single double-byte register.
While the 8051 has extensive facilities for byte logic operations as well as byte binary and two-digit BCD arithmetic, it excels in its bit handling capabilities. 128 bits in the Special Function Registers and 128 software flags in the Internal Data RAM are all supported orthogonally by the logic operations of and, or, set, clear, and complement; the conditional branch operations of jump-if-bit-set, jump-if-bit-not-set, and jump-if-bit-set- then-clear-bit; and the transfer operation of move bit. Performing conditional branch, logical, and transfer op- erations directly on Boolean variables is a breakthrough for microcomputers, since this makes the 8051 both a byte processor and a Boolean processor.
2.5.1 Data Transfer Operations
Look-up-tables resident in Program Memory can be accessed by indirect moves. A byte constant can be transferred to the A register (Le. accumulator) from the Program Memory location whose address is the sum of a base register (the PC or DPTR) and the index register
(A). This provides a convenient means for programming translation algorithms such as ASCII to seven segment conversions. The Program Memory move operations are shown diagrammatically in Figure 2. 17.
REGISTER
A
BASE-REGISTER- PLUS | BASE-REGISTER- PLUS |
INDEX-REGISTER-INDIRECT | INDEX-REGISTER-INDIRECT |
@PC+A | @OPTR+A |
(PROG MEM 0-64K) | (PROG MEM 0-64K) |
Figure 2.17 Program Memory Move Operations
A byte location within a 256-byte block of External Data Memory can be accessed using R I or RO in Register- Indirect Addressing. Any location within the full 64K External Data Memory address space can be accessed through Register-Indirect Addressing using a 16-bit bas.. register (i.e. the Data Pointer). These moves are shown in Figure 2.18.
The byte in-code-constant (immediate) moves and byte variable moves within the 8051 are highly orthogonal as detailed in Figure 2. 19. When one considers that the accumulator and the registers in the Register Banks can be Direct Addressed, the two-operand data transfer operations allow a byte to be moved between any two of the RB registers, Internal Data, RAM, accumulator and Special Function Registers. Also, immediate operands
