Xilinx UG129 manual Arithmetic Logic Unit ALU, Flags, Byte Scratchpad RAM, Input/Output

PicoBlaze Microcontroller Functional Blocks

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Other memory organizations are possible to accommodate more PicoBlaze controllers within a single FPGA or to enable interactive code updates without recompiling the FPGA design. See Chapter 7, “Instruction Storage Configurations,” for more information.

Arithmetic Logic Unit (ALU)

The byte-wide Arithmetic Logic Unit (ALU) performs all microcontroller calculations, including:

•basic arithmetic operations such as addition and subtraction

•bitwise logic operations such as AND, OR, and XOR

•arithmetic compare and bitwise test operations

•comprehensive shift and rotate operations

All operations are performed using an operand provided by any specified register (sX). The result is returned to the same specified register (sX). If an instruction requires a second operand, then the second operand is either a second register (sY) or an 8-bit immediate constant (kk).

Flags

ALU operations affect the ZERO and CARRY flags. The ZERO flag indicates when the result of the last operation resulted in zero. The CARRY flag indicates various conditions, depending on the last instruction executed.

The INTERRUPT_ENABLE flag enables the INTERRUPT input.

64-Byte Scratchpad RAM

The PicoBlaze microcontroller provides an internal general-purpose 64-byte scratchpad RAM, directly or indirectly addressable from the register file using the STORE and FETCH instructions.

The STORE instruction writes the contents of any of the 16 registers to any of the 64 RAM locations. The complementary FETCH instruction reads any of the 64 memory locations into any of the 16 registers. This allows a much greater number of variables to be held within the boundary of the processor and tends to reserve all of the I/O space for real inputs and output signals.

The six-bit scratchpad RAM address is specified either directly (ss) with an immediate constant, or indirectly using the contents of any of the 16 registers (sY). Only the lower six bits of the address are used; the address should not exceed the 00 - 3F range of the available memory.

Input/Output

The Input/Output ports extend the PicoBlaze microcontroller’s capabilities and allow the microcontroller to connect to a custom peripheral set or to other FPGA logic. The PicoBlaze microcontroller supports up to 256 input ports and 256 output ports or a combination of input/output ports. The PORT_ID output provides the port address. During an INPUT operation, the PicoBlaze microcontroller reads data from the IN_PORT port to a specified register, sX. During an OUTPUT operation, the PicoBlaze microcontroller writes the contents of a specified register, sX, to the OUT_PORT port.

See Chapter 6, “Input and Output Ports,” for more information.

PicoBlaze 8-bit Embedded Microcontroller www.xilinx.com 13

UG129 (v1.1.2) June 24, 2008