AT43USB355_06 datasheet(9/115 Pages) ATMEL

9 / 115 page

AT43USB355

2603G–USB–04/06

All register operating instructions in the instruction set have direct and single cycle access to

all registers. The only exception is the five constant arithmetic and logic instructions SBCI,

SUBI, CPI, ANDI, and ORI between a constant and a register, and the LDI instruction for load

immediate constant data. These instructions apply to the second half of the registers in the

between two registers or on a single register apply to the entire register file.

As shown in Table 1, each register is also assigned a data memory address, mapping them

directly into the first 32 locations of the user Data Space. Although not being physically imple-

mented as SRAM locations, this memory organization provides great flexibility in access of the

registers, as the X-, Y-, and Z-registers can be set to index any register in the file.

X-, Y- and Z-

Registers

Registers R26..R31 contain some added functions to their general-purpose usage. These reg-

isters are address pointers for indirect addressing of the Data Space. The three indirect

address registers X, Y, and Z are defined as:

In the different addressing modes these address registers have functions as fixed displace-

ment, automatic increment and decrement (see the descriptions for the different instructions).

ALU – Arithmetic

Logic Unit

The high-performance AVR ALU operates in direct connection with all 32 general-purpose

working registers. Within a single clock cycle, ALU operations between registers in the register

file are executed. The ALU operations are divided into three main categories – arithmetic, log-

ical and bit-functions.

Program Memory

The AT43USB355E contains 24K bytes on-chip downloadable memory for program storage

while the AT43USB355M has a masked programmable ROM. Since all instructions are 16- or

32-bit words, the program memory is organized as 12K x 16. The AT43USB355 Program

Counter (PC) is 14 bits wide, thus addressing the 12,288 program memory addresses.

Constant tables can be allocated within the entire program memory address space (see the

LPM - Load Program Memory instruction description).

X-register

R27 ($1B)

R26 ($1A)

Y-register

R29 ($1D)

R28 ($1C)

Z-register

R30 ($1F)

R31 ($1E)

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