Picoblaze Overview
EENG 2910
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
Introduction
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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
Instruction Set
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Logical Instructions
Arithmetic Instruction
Compare and test Instruction
Shift and rotate Instruction
Data Movement Instruction
Program flow control Instructions
Interrupt related Instruction
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.
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
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 sX=sY then z1 else z0;
if sX>sY 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)
Operations
Shift and Rotate Instruction
• sl0 sX – Shift a register left 1 bit and shift 0
into the LSB. sX  sX(6…0) & 0; c sX(7)
• sl1 sX – shift a register left 1 bit and shift 1
into LSB. sX  sX(6…0) & 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.
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) –RAM[sX]  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
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
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.
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)