Stack Operations, Fifo Operations | Xilinx UG129 specification

Stack Operations

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Stack Operations

Although the PicoBlaze microcontroller has a CALL/RETURN stack, it does not have a dedicated data stack. In some controller architectures, register values are preserved during subroutine calls or interrupts by pushing them or popping them onto a data stack. The equivalent operation is possible in the PicoBlaze microcontroller by reserving some locations in scratchpad RAM.

In the example shown in Figure 5-4, the my_subroutine function uses register s0. The value of register s0 is preserved onto a “stack”, which is emulated using scratchpad RAM. When the my_subroutine function completes, the preserved value of register s0 is restored from the stack.

NAMEREG sF, stack_ptr ; reserve register sF for the stack pointer

;Initialize stack pointer to location 32 in the scratchpad RAM LOAD sF, 20

my_subroutine:

;preserve register s0 CALL push_s0

;*** remainder of subroutine algorithm ***

;restore register s0

CALL pop_s0

RETURN

push_s0:

STORE s0, stack_ptr ; preserve register s0 onto “stack”

ADD stack_ptr, 01 ; increment stack pointer

RETURN

pop_s0:

SUB stack_ptr, 01 ; decrement stack pointer

FETCH s0, stack_ptr ; restore register s0 from “stack”

RETURN

Figure 5-4:Use Scratchpad RAM to Emulate PUSH and POP Stack Operations

FIFO Operations

In a similar vein, FIFOs can be created using two separate pointers into scratchpad RAM. One pointer tracks data being written into RAM; the other tracks data being read from RAM.

See also:

•“STORE sX, Operand — Write Register sX Value to Scratchpad RAM Location,” page 112.

•“FETCH sX, Operand — Read Scratchpad RAM Location to Register sX,” page 98.

PicoBlaze 8-bit Embedded Microcontroller

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UG129 (v1.1.2) June 24, 2008

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Xilinx UG129 manual Stack Operations, Fifo Operations

Contents

UG129 v1.1.2 June 24 PicoBlaze 8-bit Embedded Microcontroller User Guide Version Revision Revision History Technical Support Limitations Limited Warranty and DisclaimerLimitations Limitation of Liability Preface Acknowledgments Acknowledgments About This Guide Guide Contents Preface About This Guide Table of Contents Interrupts Performance Appendix C PicoBlaze Instruction Set and Event Reference Introduction PicoBlaze Microcontroller Features Introduction PicoBlaze Microcontroller Functional BlocksGeneral-Purpose Registers Instruction Program Store Input/Output Arithmetic Logic Unit ALUFlags Byte Scratchpad RAM CALL/RETURN Stack ResetProgram Counter PC Program Flow Control Why the PicoBlaze Microcontroller? Why the PicoBlaze Microcontroller?Why Use a Microcontroller within an FPGA? Weaknesses Strengths Signal Direction Description PicoBlaze Interface SignalsPicoBlaze Interface Signal Descriptions Readstrobe 1PicoBlaze Instruction Set alphabetical listing PicoBlaze Instruction Set Enable Interrupt Returni Disable Address Spaces Instruction 1Kx18 Direct PicoBlaze Instruction Set Processing DataLogic Instructions 3Inverting an Individual Bit Location 2Complementing a Register Value Arithmetic Instructions 616-Setting a Bit Location SUB and Subcy Subtract Instructions 10Incrementing and Decrementing a Register Multiplication 148-bit by 8-bit Multiply Routine Produces a 16-bit Product Division 168-bit by 8-bit Multiply Routine Using Hardware Multiplier 18Loading a Register with Itself Acts as a NOP Instruction No Operation NOP Test and Compare Setting and Clearing Carry Flag 22The Test Instruction Affects the Zero Flag Shift and Rotate Instructions 25Generate Parity for a Register Using the Test Instruction 5PicoBlaze Rotate Instructions Rotate Left Rotate Right SRX Program Flow Control Moving Data 6Instruction Conditional Execution Description Program Flow Control CALL/RETURN Program Flow Control UG129 v1.1.2 June 24 1Simple Interrupt Logic Interrupts 2Example Interrupt Flow Example Interrupt Flow Example Interrupt Flow 3Interrupt Timing Diagram Interrupts Indirect Addressing Scratchpad RAMAddress Modes Direct Addressing Scratchpad RAM Implementing a Look-Up Table Stack Operations Stack OperationsFifo Operations Scratchpad RAM Portid Port Input and Output Ports 1INPUT Operation and Fpga Interface Logic Input Operations Register s0 Input OperationsPORTID70 INPORT70 Input and Output Ports Applications with Few Input SourcesReadstrobe Interaction with FIFOs Output Operations Output Operations Fpga Register Simple Output Structure for Few Output Destinations 8Use Constant Directives to Declare Output Port Addresses 9Pipelining the Portid Decoding Improves Performance Pipelining for Maximum Performance Pipelining for Maximum Performance Effective Pipelining Improves Read Performance Repartitioning the Design for Maximum Performance Standard Configuration Single 1Kx18 Block RAM Instruction Storage Configurations Two PicoBlaze Microcontrollers Share a 1Kx18 Code Image Instruction Storage Configurations Distributed ROM Instead of Block RAM 6Using Distributed ROM for Instruction Memory Instruction Storage Configurations Frequency PerformanceInput Clock Frequency Predicting Executing Performance Performance Assembler PicoBlaze Development Tools PicoBlaze Development Tools Assembly ErrorsInput and Output Files Mediatronix pBlazIDE Configuring pBlazIDE for the PicoBlaze Microcontroller 4Example of How Kcpsm Source Code Converts to pBlazIDE Code Importing KCPSM3 Code into pBlazIDE Function KCPSM3 Directive PBlazIDE Directive Differences Between the KCPSM3 Assembler and pBlazIDEDirectives Differences Between the KCPSM3 Assembler and pBlazIDE PicoBlaze Development Tools KCPSM3 Module Using the PicoBlaze Microcontroller in an Fpga DesignVhdl Design Flow Using the PicoBlaze Microcontroller in an Fpga Design Connecting the Program ROM Black Box Instantiation of KCPSM3 using KCPSM3.ngc Black Box Instantiation of KCPSM3 using KCPSM3.ngcGenerating the Program ROM using progrom.coe Generating an ESC Schematic Symbol Using the PicoBlaze Microcontroller in an Fpga Design Naming or Aliasing Registers Assembler DirectivesLocating Code at a Specific Address PBlazIDE Defining ConstantsNaming the Program ROM Output File Defining I/O Ports pBlazIDE Defining I/O Ports pBlazIDE Input PortsOutput Ports 5Example of pBlazIDE Dsout Directive Input/Output Ports Custom Instruction Op-Codes Custom Instruction Op-Codes Assembler Directives Simulating PicoBlaze Code Instruction Set Simulation with pBlazIDE Edit Simulator Control ButtonsInstruction Set Simulation with pBlazIDE 2pBlazIDE Simulator Control Buttons Function Assemble Pause RunStep Over Run to Cursor Turbocharging Simulation using FPGAs Turbocharging Simulation using FPGAs Simulating PicoBlaze Code Related Materials and References Chapter Related Materials and References KCPSM3 Syntax Example Program Templates Appendix Example Program Templates PBlazIDE Syntax ADD sX, Operand -Add Operand to Register sX PicoBlaze Instruction Set and Event Reference Appendix PicoBlaze Instruction Set and Event Reference Addcy sX, Operand -Add Operand to Register sX with Carry SX, Operand Logical Bitwise and Register sX with Operand SX, Operand Logical Bitwise and Register sX with Operand Examples Table C-1CALL Instruction Conditions Description Condition Figure C-4COMPARE Operation Compare sX, Operand Compare Operand with Register sX Disable Interrupt Disable External Interrupt Input Disable Interrupt Disable External Interrupt InputEnable Interrupt Enable External Interrupt Input KCPSM3 Instruction PBlazIDE Instruction Figure C-5FETCH Operation Portid Å Operand SX Å Inport PC Å PC + Interrupt Event, When Enabled Table C-2JUMP Instruction Conditions Description Load sX, Operand Load Register sX with Operand Or sX, Operand Logical Bitwise or Register sX with Operand Or sX, Operand Logical Bitwise or Register sX with Operand Figure C-7OUTPUT Operation and Fpga Interface Logic Table C-3PicoBlaze Reset Values Resource Reset Event Effect Reset EventReset Event Table C-4RETURN Instruction Conditions Description PBlazIDE Equivalent RET, RET C, RET NC, RET Z, RET NZ Table C-6Rotate Right RR Operation RL sX Rotate Left Register sXRR sX Rotate Right Register sX Table C-5Rotate Left RL Operation Table C-7Shift Left Operations SL 0 1 X a sX Shift Left Register sXSL 0 1 X a sX Shift Left Register sX SR 0 1 X a sX Shift Right Register sX Shift Right with ‘ 0’ fill Figure C-8STORE Operation Store sX, sY SUB sX, Operand -Subtract Operand from Register sX Figure C-10SUBCY Instruction Registers sX Flags CARRY, Zero PBlazIDE Equivalent Subc Figure C-11ZERO Flag Logic for Test Instruction PicoBlaze 8-bit Embedded Microcontroller 117 XOR sX, Operand Logical Bitwise XOR Register sX with Operand Table D-1PicoBlaze Instruction Codes Instruction Codes Jump Z PicoBlaze 8-bit Embedded Microcontroller 121 Appendix Instruction Codes Reg Description Register and Scratchpad RAM Planning WorksheetsRegisters Loc Description Scratchpad RAMAppendix Register and Scratchpad RAM Planning Worksheets