Xilinx UG129 Defining Constants, Naming the Program ROM Output File, Defining I/O Ports pBlazIDE

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Chapter 11: Assembler Directives

Defining Constants

Similar to renaming registers, assign names to constant values. By defining names for constants, it is easier to understand and document the PicoBlaze code rather than using the constant values in the code. Similarly, assigning names to registers and constants simplifies code maintenance. Updating the value assigned to a constant is easier if the constant is declared just once rather than searching for each occurrence in the application code.

Table 11-3shows how to define a constant called myconstant and assign the value 80 hexadecimal. Both KCPSM3 and pBlazIDE formats are shown.

Table 11-3:Assembler Directives to Name or Alias Registers

KCPSM3

pBlazIDE

CONSTANT myconstant, 80

myconstant EQU $80

Naming the Program ROM Output File

The PicoBlaze assembler generates object code and formats the results for some form of internal memory within the FPGA. In general, the internal memory is block RAM, as described in Chapter 7, “Instruction Storage Configurations.”

KCPSM3

The output files from the KCPSM3 assembler are always named according to the source program name. For example, an assembly program named myprog.psm produces output files called myprog.vhd, myprog.v, etc. for the various output formats.

pBlazIDE

The pBlazIDE assembler provides a directive, using the keyword VHDL, to explicitly name the target output file and the VHDL entity name, as shown in Figure 11-1. The template.vhd file contains the VHDL template for the program ROM. The target.vhd file is the output VHDL file, derived from the template.vhd file, which contains the initialization values created by assembling the PicoBlaze code. Finally, entity_name is the VHDL entity name used to name program ROM.

VHDL "template.vhd", "target.vhd", "entity_name"

Figure 11-1:pBlazIDE Directive to Name the VHDL Output File

Defining I/O Ports (pBlazIDE)

To aid modeling and debugging of the interaction between the PicoBlaze microcontroller and the remainder of the FPGA, the pBlazIDE assembler supports some additional directives to describe and define I/O ports. These directives are particularly useful during instruction set simulation, as shown in Figure 12-1.

76	www.xilinx.com	PicoBlaze 8-bit Embedded Microcontroller
		UG129 (v1.1.2) June 24, 2008