Specifications

ManualsBrandsEricsson ManualsCell PhoneCH337

SERIES 3/3A PROGRAMMING GUIDE

4-2

The data displayed is updated every time

Spy

comes into foreground, and also whenever the

Update

option is selected. By default, it is also updated regularly on a timer, though this can be disabled by a

menu option. The

Refresh rate

option governs how frequently updates take place, when the timer is

enabled.

There are in all twelve pieces of data that can be displayed for each process, but only three of these can be

seen at any one time. Use the

Change data

menu option to choose which.

Many of the data items can be meaningfully displayed either in Hex or in Decimal. Another menu option

controls this.

Heap statistics

Five of the twelve possible items of data concern the allocator heap of the process. Each process has its

own heap, which can vary in size according to the needs of the program. Thus a Word Processor editing a

large document will typically have a larger heap than a Word Processor editing a smaller document.

Each heap is divided into "alloced cells" and "free cells". The items "Cells allocated" and "Cells free"

count these, and the items "Bytes allocated" and "Bytes free" sum how many bytes belong in each

category.

This data can be of great help in developing applications. It is of course vital that an application frees

cells it no longer requires - otherwise these cells go to what is called "alloc heaven". Something to watch

for in particular is alloc heaven following an out-of-memory failure. Typically, a process such as

launching a dialog involves a number of different allocs; if any one of these fails, all the allocs which have

already succeeded must be undone. System code provides mechanisms such as "automatic destruction"

and "automated clean-up" to help applications here, but applications can use these incorrectly at times -

hence the need for real-time checking.

Stack statistics

Whenever a process starts, its stack is filled up with

0xFF

's. This makes it easy to see how much stack has

been used, at any one time. Quality programs need to avoid having too large a stack - the built-in

applications default to a stack of

0xa00

. On the other hand, the operating system panics them (panic 69)

if it ever discovers that their stack is less than

0x100

. This is because whenever an interrupt occurs, it

runs in the stack of the current process.

The

Reset least stack

menu option simply refills the bottom of the process's stack with

0xFF

's (ie up to its

present stack pointer).

Segment statistics

The "Segment size" of a process gives the size of its data segment - which consists of the heap, static data

private to the application, the stack, and finally the Epoc reserved statics at the bottom end. The quoted

"Segment size" of an application can sometimes give a misleading account of how much memory it is

actually using - since there are free cells as well as alloced cells in the heap. From time to time, the

operating system may try to compress these heaps, but it can only do this by removing any free cells

at the

end

of the heap. Applications should strive to avoid ending up with large free cells in the middle of their

heap - though this is a very difficult goal to achieve.

Tests for heap integrity

Whenever

Spy

collects heap statistics for an application, it also checks the heap integrity. Any defect

(caused for example by writing beyond the end of an alloc cell, or freeing a cell that was never alloced)

results in an immediate alert. This alert helps to pin-point problems which would otherwise only rear

their head much later - long after the real damage has been done.

Process priorities

The Process Priority gives the pecking order of the processes, as regards gaining CPU from the multi-

tasking scheduler. Most applications the user sees run at

0x80

when in foreground, and at

0x70

when in

background. This prevents computationally busy background tasks from detracting from the performance

of the foreground task.

Spy

momentarily ups its own priority to a massive

0xc0

(the maximum allowed to non-OS processes)

whenever it collects heap statistics from other processes, to lessen the chances of other processes

manipulating their heaps at the same time as

Spy

is walking through them. Occasionally,

Spy

will find

that a heap is momentarily marked as "locked" when it tries to survey it - this indicating that the

Operating System is busy doing something there - in which case the heap statistics will all just be shown

for that process.