Book - Secrets ZX-Spectrum: memory usage.

<b>Book</b> - Secrets ZX-Spectrum: memory usage.

   Chapter 5.


   And the S P O R L O W B A H & E

   N A M I T I



   After turning on the computer in the network launches 
automatically placed in the ROM at 0. It checks the amount of 
allowable memory divides it into different blocks and assigns a 
system variable initial values.



   The diagram below describes
section of memory. Volume from 0 to 16383 is located in ROM, 
the rest in the RAM. Only some units have a permanent position, 
while others may move in memory and their current addresses are 
stored in the relevant system variables. The numbers at the end 
Some areas represent pointers the end of the field and are 
always present there. 

 ______________ _________________________
| | | |
| ADDRESS | AREA | END |
|______________|______________|__________|
| | | |
| # 0000 (0) | System | |
| | Procedure | |
| # 3000 (15616) | Prototypes | |
| | Characters | |
| | Codes 32-127 | |
| # 4000 (16384) | Screen | |
| # 5800 (22528) | Properties | |
| # 5B00 (23296) | buffer principle-| |
| | Tera | |
| # 5C00 (23552) | System | |
| | Variables | |
| # 5C86 (23734) | Map micro-| |
| | Drive | |
| CHANS | Information | # 80 (128) |
| | Channel | |
|--------------|--------------|----------|
| PROG | B E J P & K | |
|--------------|--------------|----------|
| VARS | Variables | # 80 (128) |
| | ZX-BASIC | |
|--------------|--------------|----------|
| E_LINE | buffer | # 80 (128) |
| | Editor | |
|--------------|--------------|----------|
| WORKSP | Buffer inst-| # 0D (13) |
| | Struction INPUT | |
|--------------|--------------|----------|
| | Working | |
| | Region | |
| | ZX-BASIC | |
|--------------|--------------|----------|
| STKBOT | Stack tracing-| |
| STKEND | oscillator | |
|--------------|--------------|----------|
| SP | Free | |
| | Region | |
| | Memory | |
| | BASIC | |
|--------------|--------------|----------|
| ERR_SP | Machine | |
| | Stack | |
|--------------|--------------|----------|
| RAMTOP | Stack Address | # 3E (62) |
| | Return | |
| | GOSUB | |
|--------------|--------------|----------|
| | Free | |
| | Region | |
| | Memory | |
|--------------|--------------|----------|
| UDG | Prototypes | |
| | Characters, | |
| | Certain | |
| | Users-| |
| | Field | |
| P_RAMT | | |
|______________|______________|__________|



   5. 1 images of characters


   Form of printed characters with codes from 32
127 (8 bytes for each character) fill
768 recent cell ROM. They are coded
in the same form as the characters are defined by the user. 
Signs of a mosaic graphics (Codes from 128 to 143) are 
constructed each times. Start address this area 256 is stored 
in the system variable CHARS. 



   5. 2 Screen


   This area is intended for storage
information about each point of the screen, or rather
about whether she should be in color ink
or background. 6144 bytes in this block allow
address 49152 different points (1 byte
describes 8 points). They are arranged in
table of 192 rows and 256 columns. For
graphic designs BASIC available
Only the top 176 rows. Each graphic symbol is displayed in the 
field of 8x8 pixels. This allows you to display text in the 
format 24 rows of 32 positions. The bottom two text

rows are reserved for system messages and workspace editor.


   Interesting way in which this region
transferred to the screen. Unfortunately, successive 32-byte do 
not describe the sequence of image line on the screen, 256 
consecutive bytes do not describe operator of the text. Placing 
graphics line (line) is absolutely the opposite, and

in terms of ZX-BASIC similar to the invention a madman. This 
may best be seen when reading the screen with tape. Here we 
confine ourselves to examples that allow to count text and 
image coordinates on the screen address. 


   Byte containing a graphical point of
coordinates (K, M) (point (0, 0) lies in
the top left corner of the screen above the two lowest
text operators) on the screen has
Address:


   16 384 +32 * (INT ((175-M)
/ 8-INT ((175-M/64 * 8 +8 * (176 * M-INT ((175-M)
/ 8) * 8 +64 * INT ((175-M) / 64) + INT (K / 8).


   The position of bits in a byte is calculated
as


   8-K + INT (K / 8) * 8.


   In turn, the addresses of text characters
count on the model:


   16384 +2048 * INT (K / 8) +32 * (K-8 * INT (K / 8))
+256 * M + N,


   where K denotes the number operator (0
to 23), N is the number of columns (0 ...
31), in turn, M is the number
graphic lines (lines), forming
long symbol (0 ... 7). The zero is taken, lying on top of the 
block mark the line. Origin for instructions, writing signs 
placed in the top left corner of the screen. 


   5. 3 Attributes (22,528 ... 23,295)


   In this area, information is stored
placing flowers on the screen. Lowest
unit surface, the color of which can be
modify, a field unit
mark the size of 8x8 pixels. For such a field
You can define a background color, ink, blinking. A choice of 8 
colors available: 


   0 - black

   1 - Purple

   2 - red

   3 - blue

   4 - green

   5 - Blue

   6 - Yellow

   7 - white


   Each color can act in two colors: regular and intense 
(BRIGHT 0 1).


   All information concerning the identity
sign field with encrypted in a single byte:

    _______________________________

   | | |

   | Bits | Value |

   |________|______________________|

   | | |

   | 7 | Blink |

   |--------|----------------------|

   | 6 | background intensity |

   |--------|----------------------|

   | 5 | |

   | 4 | Background color |

   | 3 | |

   |--------|----------------------|

   | 2 | |

   | 1 | Color Ink |

   | 0 | |

   |________|______________________|


   To set the desired attributes of a field
and determine what value should be placed in the corresponding 
byte is better to use an example: 


   128 * F +64 * B +32 * P + I,


   where F - determines flicker (FLASH 0,
1)

   B - specifies the background color (BRIGHT 0
1)

   P - is the number of background colors

   I - is the color of ink


   Specify the address byte, described
attributes of the sign field with coordinates
K, M, is calculated as follows:


   22 528 +32 * K + M.


   5. 4 Buffer printer


   In this area, to collect data that will be sent to the 
printer. Print, in fact, follows the completion of this Buffer 
(256 bytes or 32 characters), or after the symbol "end of line" 
(CHR $ 13). If printer is not connected, then the region will 
not used by the system and can be used for other purposes.




   5. 5 Card mikrodrayva


   This area is used only when connected to SPECTRUM 
ZX-INTERFACE-1. The "naked" computer, this card is not there 
physically and CHANS = 23734. 



   5. 6 The ZX-BASIC (PROG to
VARS-1)


   After system initialization, without external devices, this 
unit starts at address # 5CC8 (23,755). It BASIC program text 
is stored. Private strings in computer memory have the following

form:

    ___________________________________

   | | | | |

   | 2 bytes | 2 bytes | Length | 1 byte |

   | | | Text | |

   |-------|--------|------|-----------|

   | Number | Length | | # 0D (13) |

   | String | text +1 | TEXT | Code <ENTER> |

   |_______|________|______|___________|


   In the case of a line number deviation from the rules and 
the first here is a byte, and then junior. All the keywords 
that appear in the text are encoded by single characters. 


   It is an interesting way to store numbers in the program. 
The sequence of characters representing the sequence of digits, 
always placed a control character CHR $ 14, followed by 5 bytes 
containing the binary representation of the numbers. During the 
program output on the screen, These 6 additional bytes are 
omitted. This explains why some programmers prefer to write VAL 
"17" instead of just 17. The first of them actually zanimaet 5

bytes, and the second 8. We are talking about saving
memory. During the execution of the program,
turn, passed the characters that recorded the number and uses 5
byte binary representation. This allows you to hide the correct 
value of certain numbers (such as starting address procedures 
in machine code) from the "curious". In this case, you must 
determine the address internal representation of the number and

POKE instructions specify its desired
value. In the derivation of the text on the screen and
will continue to appear old
representation of the number is no longer associated with the
written him a binary number! Applying this technique must be 
remembered that each extract a row at the bottom

of the screen and return it to the place, even
without any modifications, changes the internal representation 
of each number in the text, assigning them the values visible 
on the screen. 


   Among other interesting tricks give
two more. If the first line of the program,
cells containing the number, put
number 0, then an operator can not be copied to the bottom of 
the screen editor area, and then modify it.

At the other end of the program possibly other
Gizmo. There's update line 9999 REM and after establishing its 
address in both bytes containing the number, place value

255. The first result of this will be borne by
program output to the screen. We modified the string does not 
appear. The second effect is much more valuable, there will be 
at try reading the program from a tape instruction MERGE. 
SPECTRUM offended user-friendly and does not respond to

any keys.


   5. 7 variables ZX-BASIC (VARS to
E_LINE-2)


   In this area, the system stores all the variables that are 
initialized as a program or via the keyboard. An alternating 
sequence appended at the end of this area (all subsequent 
blocks will automatically shift) and remain there until the 
area is not cleared. Exceptions are simple text variables: when 
some of them are modified, and removed the previous version 
(which is accompanied by the necessary movement of other blocks 
of memory), and the new appended at the end of this area. Then

the same happens in the new array declaration.


   Storage method depends on variables
types. In the ZX-BASIC there are 6 different types of 
variables, denoted by numbers from 2 to 7: 


   2 (010) - simple text variables;

   3 (011) - a simple numeric variable

   odnoliternymi with names;

   4 (100) - numeric arrays;

   5 (101) - a simple numeric variable

   mnogoliternymi with names;

   6 (110) - landmark arrays;

   7 (111) - variables that control the cycles

   FOR ... NEXT.


   In all cases the first byte description
variable contains the type and number
first characters name. Method of placement
this information is in one word, as follows:

    ____________________________________

   | | | |

   | | Type | Number of characters |

   |----------|--------|----------------|

   | Bit | 7 6 5 | 4 3 2 1 0 |

   |__________|________|________________|


   Note that bits 4 ... 0 contain the serial number of the 
letters in the alphabet, but not its code. Own code, we obtain

adding to the number # 60 (96). Keeping the names of
SPECTRUM automatically replaces the variables
big letters small, and ignores all
gaps. The contents of the following bytes depends on the type 
of variable. 


   A simple text variable length
description of N + 3 bytes:

    ___________________________________

   | | | |

   | Type and | N = length of the text | text |

   | Literature | (2 bytes) | |

   |________|__________________|_______|



   Simple numeric variable with odnoliternym name, description 
length 6 bytes: 

    ____________________________________

   | | |

   | Type and literature | Contents (5 bytes) |

   |______________|_____________________|



   Numeric array description length N + 3
bytes:

  ______________________________________
 | | | | | |
 | Type | N = length | K = number | 1-th ... K-th | E-you |
 | And | description-| index-| index | of |
 | Literature | of 3 | owls | (for 2 b.) | 5 b. |
 | | (2 b.) | | | |
 |______|_______|_______|_________|_____|



   Simple numeric variable with mnogoliternym name, description 
length K + 5 bytes 

 ________________________________________
| | | |
| Type and literature | 2nd sign ... K-th symbol | The values-|
| | Name name | chenie |
| | | 5 b. |
|____________|_____________________|_____|



   Second and subsequent name characters are stored as codes of 
the characters. The last byte is the name of the eldest

bits are always set to 1, which allows you to always recognize 
it. 


   Landmark array, the length of the description of N + 3
bytes:
 ________________________________________
| | | | | |
| Type and | N = length | K = number | 1-th ... K-th | E-you |
| Literature | description-| index-| index | of |
| | Of 3 | owls | | |
| | (2 b.) | | (At 2 b.) | (1 b.) |
|______|_______|________|_________|______|



   Driven cycles of variable length
descriptions of 19 bytes:
 ________________________________________
| | | | | | |
| Type and | The value-| The value-| Step = Z | K = | Number |
| Literature-| of | of | | Number | FOR in |
| Pa | current-| finite-| | line | row-|
| | Schee = X | Noe = Y | | | Ke +1 |
| | (5 b.) | (5 b.) | (5 b.) | (2 b.) | (1 b.) |
|_____|______|______|______|______|______|



   The last byte of the field variables as
pointer always contains the value # 80
(128).


   5. 8 buffer editor (E_LINE to
WORKSP-1)


   In this region, hosted a program line or sequence of 
directives to direct performance during their

keyboard input. Input characters are copied to the bottom of 
the screen. After clicking <ENTER> computer checks the fidelity 
Text and take appropriate action. 



   5. 9 Buffer instructions INPUT (WORKSP to
STKBOT-1)


   In this buffer is initially introduced
data read command INPUT and only after pressing <ENTER> should 
convert them. In this area, as SPECTRUM performs some 
operations that require working memory (eg, coupling of 
symbolic variables). 



   5. 10 Stack Calculator (STKBOT to
STKEND-1)


   Workspace system routines
perform most arithmetic
operations.



   5. 11 Free Region of BASIC
(STKEND +1 to SP)


   This memory block is free, but is designed for the needs of 
BASIC. Working areas at both ends of the block replenished at 
its expense. The data posted here may be subject to destruction 
without warning. Before the onset of action, requiring 
increased some of the working units, SPECTRUM checks whether 
there will be in this unit at least 80 free bytes on the 
possible needs of the machine stack. A negative test result

signaled by the message 4. Address of the end
this field is stored in the register of the processor Z80, 
called the stack pointer (SP) and available from the BASIC 
system. 



   5. 12 Stack Machine (SP before ERR_SP)


   The stack is used for processor Z80
current storage of various data, address, etc. This area is 
absolutely useless for programming in BASIC. 



   5. 13 GOSUB stack return addresses
(ERR_SP +1 to RAMTOP)


   Another stack, this time for storing the line number and 
position in her statement GOSUB. These data needed for the team 
RETURN when non-emergency exit from the subroutine. Reusable 
performance or GOSUB RETURN causes shift the machine stack, 
addressable through ERR_SP to 3 bytes up or down. Therefore,

modifications recommended by a special unit
caution.


   5. 14 areas of free memory
(RAMTOP +1 to P_RAMT)


   The block is really free memory and
secure in the sense that its contents are not available for 
BASIC system and can be modified only upon request

user by using POKE.
Even directive NEW does not violate this area. In principle, it 
is used to store programs in machine code or country-specific 
data for which BASIC variables are not suitable. 



   5. 15 Images of characters user (UDG
to UDG +167)


   After system initialization, the block
begins with a cell having an address
RAMTOP +1 = # FF58 = 65,368, and ends in
P_RAMT = # FFFF = 65535. The address of this block
Memory can be obtained by performing manual
USR "A". This area is used for storage
outlines of graphics, user-defined. Determination of the unit
symbol consists of eight bytes, describing the state of each 
point in the sign field size 8 * 8.



   For example, define a graphic symbol and place it in the UDG 
so that he displayed after pressing the mode <O> G. The first 
step is design form a new label in the box 8 * 8. In the field, 
intended to paint, placed 1, and let 0. Each filled so the 
operator is treated as 8-digit binary number. The resulting set 
of 8 numbers describes the shape of the projected symbol:



   0 0 0 0 0 1 0 0 4

   0 0 0 0 1 0 0 0 8

   0 0 0 0 0 0 0 0 0

   0 0 1 1 1 1 0 0 60

   0 1 0 0 0 0 1 0 66

   0 1 0 0 0 0 1 0 66

   0 0 1 1 1 1 0 0 60

   0 0 0 0 0 0 0 0 0


   Now we introduce it into the computer. Simply
just do it with the help of the program:


   1910 DATA 4, 8, 0, 60, 66, 66, 60, 0

   20 FOR I = USR "0" TO USR "0" + 7

   30 READ A: POKE T, A

   40 NEXT I


   After her performance in key <O> G mode displays a coded 
symbol. In a chain of characters it will have code 158. The 
data in row 10, you can apply also in binary instead of 
decimal, using the BIN. This requires more writings, but it is 
much easier all the modifications that we determined the 
sample. 


   168 bytes of UDG allow you to store
up to 21 different new character. Their codes
placed in the range from 144 (A) to 164
(U). After system initialization, this
block contains sample forms large letters
Latin alphabet.