Picoblaze Overview

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Picoblaze Overview

• EENG 2910

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Introduction

• 8-bit microcontroller for Xilinx devices
• Soft Core Soft Processor
• 5 of the resources of spartan 3 (3S200 devicee)
• Compact, and Flexible
• Utilized for simple data processing and control

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Introduction

• 8-bit data width
• 8-bit ALU with carry and zero flag
• 16 8-bit general-purpose registers
• 64-byte data memory
• 18 bit instruction width
• 10-bit instruction address (supports 1024 instr.)
• 256 input ports and 256 output ports
• 2 clock cycles per instruction

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Instruction Set

• Logical Instructions
• Arithmetic Instruction
• Compare and test Instruction
• Shift and rotate Instruction
• Data Movement Instruction
• Program flow control Instructions
• Interrupt related Instruction

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Programming Model (Notation)

• sX, sY represents General Purpose Register
• Pc Program Counter
• Tos top-of-stack pointer of the call/return
  stack
• c, z, i carry, zero, and interrupt flags
• Kk 8 bit constant or port id
• Ss 6-bit constant data memory address
• AAA 10-bit constant instruction memory address.

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Instruction Format

• op sX, sY register-register format (sX, sY
  operand, op operation, sX destination, sX ?
  sX op kk)
• op sX, kk register-constant format (?)
• op sX single register format used in
  shifting and rotating instructions with one
  operand.
• op AAA single-address format. E.g jump and call
  instruction. AAA address.
• Op - instruction without operand

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Operations (Examples)

• Logical Operation (6 instructions) and, or, xor
• Arithmetic operation (8) add, addcy or addc
  (addcy add with carry flag), sub, subcy or
  subc.
• Compare and Test operation compare sX, sY or
  comp sX, sY , compare sX, kk or comp sX, kk
• if sXsY then z?1 else z?0
• if sXgtsY then c?1 else c?0
• test sX, SY
• t ?s X and s Y
• if t 0 then else z? 0
• C ? t(7) xor t(6) xor ....xor t(0)

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Operations

• Shift and Rotate Instruction
• sl0 sX Shift a register left 1 bit and shift 0
  into the LSB. sX ? sX(60) 0 c ?sX(7)
• sl1 sX shift a register left 1 bit and shift 1
  into LSB. sX ? sX(60) sX(0) c ? sX(7)
• slx sX shift a register left 1 bit and shift
  sX(0) into the LSB.
• sla sX shift left 1 bit and shift c into LSB
• similarly, sr0, sr1, srx, sra sX, rl sX, rr sX
  etc.

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Data Movement

• Computation done via Registers and ALU
• Movement between
• - registers load
• Register and data RAM Fetch and Store
• Register and I/O port input and output
• Load sX, sY load sX, kk - sX ? (sY or kk)
• fetch sX, (sY) or fetch (sX, sY)- sX ? RAM(sY)
• Fetch sX, SS, - sX ? RAM(sY)
• store sX, (sY) (or store sX, sY) RAMsX ? sX -
  move data from register to data RAM.
• Input sX, (sY), or in sX, sY move data from
  input port to a register. sX ? in_port
• Output sX, (sY) (out sX, sY) move data from a
  register to the input port

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Program Flow

• Jump, call, and return load a value to the
  program counter and modify program flow.
• Unconditionally or conditionally based on zero
  and carry flags.
• Jump AAA unconditional jump
• Jump c, AAA jump if carry flag is set ( if c
  0 then pc ? AAA else pc pc 1)
• Jump nc, AAA jump z, AAA jump nz, AAA

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Program flow - Call and Return

• Implementation of functions
• Call loads a new value to the pc, saves it
  current value to a stack (special buffer).
• The routine contains a return( obtains the saved
  value from the stack and increment by 1) at the
  end .
• Picoblaze support nested function (LIFO) .
  Picoblaze has 31-word stack.

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Program flow

• Call AAA (unconditionally call subroutine)
• tos ? tos
• Stack tos ? pc
• Pc ? AAA
• Call c, AAA (conditionally call subroutine)
• If (c 1)
• tos ? tos
• Stack tos ? pc
• Pc ? AAA
• Else
• Pc ? pc 1
• Return (ret)
• Return c, (ret c)