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Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Microcomputers
Pointers, Subroutines, and Stack
Memory Usage
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Arrays and Pointers in C
• A pointer variable is a variable that contains the
address of another variable.
• An array is a collection of like elements, such as an
array of integers, array of characters, etc.
• One use of pointer variables in C is for stepping
through the elements of an array.
• Another use of pointer variables is for passing
arrays to subroutines
– Only have to pass the address of the first element instead
of passing all of the array elements!
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
A First Look at C Pointers
& is “address of” operator, “*” is dereference operator.
Pointers to data RAM are 16-bits wide because there are 64Ki
addressable locations.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Same Example with uint32 variables
p is still 16-bits! Pointer
size is always 16 bits, not
dependent upon referenced
Copyright Delmar Cengage Learning 2008. All Rights Reserved.From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”. data size
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
uint32* Pointer Example to Assembly
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”. Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Indirect Addressing Modes
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Indirect Addressing Modes Examples
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”. Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Register Offset Examples
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Add two uint16 Arrays
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”. Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
C Strings
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
The repeat Instruction
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”. Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Why are Subroutines needed?
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
A Stack
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”. Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Push/Pop on PIC24 Stack
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Push/Pop Forms
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”. Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Push/Pop Example
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6- Dynamic Allocation for Locals Dynamic allocation is needed for recursive functions to operate correctly. New space for parameters and locals are allocated in registers or on the stack. Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6- Rules For Subroutine Parameter Passing Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”.
- W0-W7 are used for parameters, left to right order. W0-W7 are caller saved (if caller wants these preserved, caller has to save them).
- Function values returned in W0-W3 (W0 for 8/16 bit, W0-W for 32-bit, W0-W3 for 64-bit values).
- Registers W8-W14 are callee saved (if the callee uses them, must be perserved).
- Locals are allocated to unused W0-W7 registers, and also to W8- W14.
- 8-bit, 16-bit return values are returned in W0; 32-bit values in W0 (LSW), W1 (MSW); and 64-bit values in W0-W3.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Subroutine Example
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”. Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Subroutine Call Example
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Global Variable Initialization
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
What does ‘init_variables’ do?
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”.
• Initial values for variables live in program memory which is
non-volatile.
• Can use a special mode called program space visibility (PSV)
that allows upper half of memory to be mapped to program
memory.
• Can then use instructions to copy data from program memory to
data memory to initialize variables.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Init_variables
(1) .text ;program memory
(2) ;; constant data to be moved to data memory
(3) sz_1_const: .asciz "Hello"
(4) sz_2_const: .asciz "UPPER/lower"
(5) init_variables:
(6) ;turn on program visibility space, use default PSVPAG value of 0
(7) bset CORCON,#2 ;enable PSV
(8) ;copy source address in program memory to W
(9) mov #psvoffset(sz_1_const),W
(10) mov #sz_1,W3 ;destination address in data memory
(11) rcall copy_cstring
(12);copy source address in program memory to W
(13) mov #psvoffset(sz_2_const),W
(14) mov #sz_2,W3 ;destination address in data memory
(15) rcall copy_cstring
(16) return
(17);;copy constant null-terminated string from program memory to data
memory
(18);;W2 points to program memory, W3 to data memory
(19) copy_cstring:
(20) mov.b [W2],W
(21) cp.b W0,#0 ;test for null byte
(22) bra Z, copy_cstring_exit ;exit if null byte
(23) mov.b [W2++],[W3++] ;copy byte
(24) bra copy_cstring ;loop to top
(25) copy_cstring_exit:
(26) mov.b [W2++],[W3++] ;copy null byte
(27) return
Copy strings
from
program
memory to
data memory
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Stack Frames
Used when cannot fit locals, parameters in registers.
Use the stack for storage.
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
Calling fib()
Copyright Delmar Cengage Learning 2008. All Rights Reserved. From: Reese/Bruce/Jones, “Microcontrollers: From Assembly to C with the PIC24 Family”. Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
fib() Implementation
fib() with stack
frames required
20 instructions,
without stack
frames only 15
instructions.
The generality
of stack frames
has overhead
costs.
Electrical & Computer Engineering – Microcomputers Dr. Jeff Jackson Lecture 6-
What do you have to know?
• Indirect addressing modes for PIC
• Implementation of C code with pointers in PIC
assembly.
• How subroutine call/return works
• How the stack on the PIC24 works
• How to pass parameters to a subroutine using
registers for parameters and locals
• How to access data in program memory
• What are stack frames used for?
