UEE072HM

1
UEE072HM

• Linking HLL and ALP
• An example on ARM

2
Embedded and Real-Time Systems

• We will mainly look at embedded systems
• Systems which have the computer system embedded
  within their application area, normally using a
  specialised single board computer (SBC)
• There is a difference between embedded and
  real-time one does not imply the other
• Hard real-time vs soft real-time

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Embedded Systems Development

• Embedded systems require host-target development
  this means
• The development is done on a different machine
  than the target
• The code must be downloaded to run
• Can be running on different CPU and architecture

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Host Target Development
Target boards
Development Host (Linux?)
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Embedded Systems Development

• Embedded systems development requires
• Mixed language programming HLL plus assembler
• Increased knowledge of tools compiler, linker
  and librarians
• Use of specialised tools i.e. down loaders
• Special skills to debug code

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Using C for Embedded Systems
define ADDRESS 0xFFFF41 main() char
mem_p mem_p(char)ADDRESS while !(mem_p
) // do nothing

• A major requirement is for direct memory access,
  this is achieved using pointers.
• This style of code is quite common

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Using C for Embedded Systems

• Note
• Use of define
• Use of char
• Use of volatile
• Casting of return value
• Use of binary op

define ADDRESS 0xFFFF41 define MASK
0x3 main() volatile char mem_p mem_p(char
)ADDRESS while !(mem_p MASK ) // do
nothing
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Using C for Embedded Systems

• Using C in Embedded Systems you will need to know
  more about
• Compilation process
• Linking
• Storage issues

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Using C for Embedded Systems

• Compilation issues
• Passing parameters
• Naming conventions
• Symbol table and map generation
• Assembler output

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Why use HLL?

• Why do high level language programmers need to
  know the low level details of storage and
  function calls?
• To write more efficient code
• To write assembler routines that are called from
  a HLL
• To call assembler routine from a HLL
• Improves knowledge of both tools and CPU

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Why use ALP?

• Why use assembler in place of HLL?
• Speed up code
• Smaller code
• Special instructions
• Boot strap routines
• Optimise HLL code

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Using C for Embedded Systems

• Ways to link HLL ALP - passing parameters
• Parameter blocks
• No recursion
• Using registers
• Limited numbers of registers
• Using the stack
• Most favoured method
• Many use a combination of the above

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The run-time stack

• In order to see how HLLs are implemented we need
  to know about the run-time stack and the
  registers that control it.
• The stack is used for
• Storing important values that may get changed
  during execution
• Local variables
• Passing some function parameters

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Function set-up

• In C function set up consists of
• Creating an executable label
• Saving the stack pointer into ip
• Storing registers used onto the stack
• Using stmfd store multiple registers instructions
• Putting the frame pointer at the stack base

gcc2_compiled..text .align 2 .global main .ty
pe main,functionmain _at_ args 0, pretend 0,
frame 8 _at_ frame_needed 1 mov ip,
sp stmfd sp!, fp, ip, lr, pc sub fp, ip, 4
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Function set-up
Stack
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Local variables

• The stack is used for local variable storage
• The compiler creates a stack frame to store the
  variables
• The variables are accessed by an offset from the
  frame pointer fp
• This means
• variable names can be repeated
• functions can be recursively called

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Creating a stack frame
Create the Stack frame
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A stack frame
Registers
Low memory
SP Stack Pointer
j
20
FP - 20
i
10
FP - 16
PC
LR
IP
FP Frame Pointer
FP
High memory
IP Intra-Procedure
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Leaving a function call

• The code to leave a function call is simple
• ldmea
• Load multiple registers command is used from the
  stack
• The old
• frame stack pointers program counter
• are restored
• This does a return with a cleaned stack

.L2 ldmea fp, fp, sp, pc.Lfe1 .size
main,.Lfe1-main
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_at_ Generated by gcc 2.95.3 20010315 (release) for
ARM/elf .file "ex1.c"_at_ GNU C version 2.95.3
20010315 (release) (arm-linux) compiled by GNU C
version 2.95.2 20000220 (Debian GNU/Linux)._at_
options passed -fverbose-asm_at_ options enabled
-fpeephole -ffunction-cse -fkeep-static-consts_at_
-freg-struct-return -fsjlj-exceptions -fcommon
-fverbose-asm -fgnu-linker_at_ -fargument-alias
-fident -mapcs-32 -mshort-load-bytes
-mno-short-load-wordsgcc2_compiled..text .alig
n 2 .global main .type main,functionmain _at_
args 0, pretend 0, frame 8 _at_ frame_needed
1, current_function_anonymous_args 0 mov ip,
sp stmfd sp!, fp, ip, lr, pc sub fp, ip,
4 sub sp, sp, 8 mov r3, 10 str r3, fp,
-16 mov r3, 20 str r3, fp, -20 mov r0,
1 mov r1, 2 mov r2, 3 mov r3,
4 bl do_it.L2 ldmea fp, fp, sp,
pc.Lfe1 .size main,.Lfe1-main