Chapter 6.
C & S T E M E N Li
P E R E M E N N E Li
System variables are stored at address
# 5C00 (23552) to the region occupied by the BASIC program.
Program from the ROM use it to store information, describing
the state of the computer. Below we consider those variables
that can be modified and used programmer. The rest is better
not to touch, as this can cause damage to the operating system.
Describes the variables divide (according to specific functions,
instead of in memory) into 5 groups:
- Storing important for a system of addresses;
- Serving the keyboard;
- Describing the state of the system;
- The channels and streams;
- Serving the television screen.
6. 1 Important for the system address
ERR_SP # 5C3D (23613) STKEND # 5C65 (23653)
VARS # 5C4B (23627) RAMTOP # 5CB2 (23730)
PROG # 5C53 (23635) P_RAMT # 5CB4 (23732)
E_LINE # 5C59 (23641) CHANS # 5C36 (23606)
WORKSP # 5C61 (23649) UDG # 5C7B (23675)
STKBOT # 5C63 (23651)
Application of these variables is described in
previous chapter. Natural use of these variables can be
redistributed size of free memory, or busy program, variables,
etc. Only variable RAMTOP can be easily
modified from BASIC through
CLEAR K. Unfortunately, this team has
well as other side effects that sometimes makes it impossible
to use.
P_RAMT
Contains the address of the last, physically
present in computer memory. Consequently, it can be used
for the recognition program in a computer model ZX-SPECTRUM it
is loaded (16K or 48K).
CHARS
With this variable you can define your own form of
withdrawal. After placing them in the RAM at address K is
sufficient to specify the value of the variable CHARS K-256.
From this point on, all text, including a program listing will
be printed in our new signs. This way you can change the shape
of all the characters with codes from 32 to 127. It should be
emphasized that both modified only the outlines of characters
but not their values, which depend on the character codes. You
can use this variable tricks that make the text of the program
is absolutely unreadable. Enough to reduce the value of CHARS
at 8, and then a printout of the program will be difficult to
find any meaning. But the inscription, the output is running on
the execution program will remain legible when
will advance shifted by one
code value, ie, in the original text will place "B" instead of
"A", "O" instead of "N", etc. etc..
UDG
The benefit of placing the address in
list for the variables is that
this unit we can move in memory.
Equally valuable is the ability to use multiple sets of atypical
characters simultaneously. In this case, before the conclusion
of the sign is required to modify the UDG, assigning it a value
set that is needed in the moment. Modifying the other variables
this group is very dangerous and can easily crash and the crash
of the operating system.
ERR_SP
This variable specifies the base of the machine stack.
There, in turn, put the address of procedure in the memory ROM,
responsible for the maintenance of all error situations that
require interruption of the program and issue the appropriate
message. By mistake is here understood as normal and completion
of the program, signaled message 0 OK. Passionate programmers
Z80 assembler can know, thanks
this, write your own error-handling code, which allows, among
other things, adding new commands to the language. Modification
ERR_SP with BASIC level always leads to system crash at the
time of occurrence of situations that require the withdrawal of
any message. It is often used to protect software from
unauthorized viewing and copying. This method in conjunction
with recording programs using SAVE F LINE ..., and also with
the exception of the possibility of reading program MERGE
statement typically can fully protect the program from less
literate pirates. Unfortunately, the salvation of the many
programs do not replicate.
6. 2 variables serving keyboard
KSTATE # 5C00 (23552)
This variable is constructed from 8 bytes
used to read the keyboard and services samopovtoryaemosti keys.
For programmer only the cell with address # 5C04
(23,556) can make a difference. It contains
255, if no key is pressed, or code of the basic values down
keys in C (large alphabet letter or number). PEEK 23556 gives so
the same effect as the CODE INKEY $ in
Mode C. The advantage is the fact that
value does not depend on the state of the cursor
(L or C). In the case of clicking a few
keys simultaneously will always recognizes the first (INKEY $
in such a situation can ignore). Keys and ,
clickable separately, would not affect
our cell, but pressed together give the code
14. In addition, the combination , ,
, Give out codes accordingly
15, 4, 5, 6 (these codes also catches CODE
INKEY $).
LAST_K # 5C08 (23560)
This variable holds the byte code of the last key pressed,
regardless of the clicks on it or not. In the case of clicking
a few - will be remembered the code first. In contrast to the
previous variable LAST_K monitors the state of the cursor. C
This variable partly related to the following
variable.
FLAGS # 5C3B (23611)
How many times a variable takes LAST_K
new value, as many times as the fifth variable FLAGS bits set
to 1. This important if we wish to recognize repeatedly
pressing the same key. In turn, the third bit of the variable,
together with the contents of MODE, allows us to identify in
what situation regarding reading the keyboard should be used
cursor K. However, this information may be used only with the
level of assembly. 7 bits FLAGS variable signal system
that came to execute commands with the keyboard or from the
program. Placed in the program instruction POKE 23611, 0 abort
her execution of message 0 OK.
REPDEL # 5C09 (23651) and REPPER # 5C0A
(23652)
Two of these variables must be considered simultaneously.
The first of them determines how long you press the button to
earn auto-repeat, and the second determines the time between
reading from the keyboard. In both variables, time is given in
1 / 50 seconds. On top of these variables are initialized to 35
and 5, which indicates that a keystroke after 0. 7 sec begins
to replicate at a rate of about 10 characters per second. In
the process of reading from the keyboard, these values can be
reduced to 0. Giving them a value of 0, achieve the highest
stop, continue as same as PAUSE 256. Entering into these
cell value of 1 lead to the fact that the work of SPECTRUM is
difficult, but possible (enter any directive from the keyboard
will require the operator excellent response and sensitivity in
the fingers).
RASP # 5C38 (23608) and PIP # 5C39 (23609)
These variables define the sound that accompanies the data
entry. RASP sets the duration (in 1 / 50 sec) warning grunt
when SPECTRUM no longer wishes to read the data. Initial value
RASP = 64 (1. 26 sec). PIP defines the duration of sound, is
confirmed by pressing keys. The initial value of PIP = 0. Worth
remember that the RASP and PIP, in contrast to
most of the other system variables,
not re-initialized instruction
NEW.
6. 3 variables of the system
These variables allow the system to monitor the situation
and correctly interpret the next program line.
NEWPPC # 5C42 (23618) and NSPPC # 5C44
(23620)
The first of them (2 bytes) contains a number
line and the second (1 byte) makes a transition. The following
commands cause a direct transition to the M-th instruction in
the L-rd line, therefore, are much more accurate command GOTO L.
POKE 23618, L-256 * (INT (L/256)):
POKE 23619, INT (L/256):
POKE 23620, M
PPC # 5C45 (23621) and SUBPPC # 5C47
(23623)
These variables contain, respectively,
row and the command being executed.
Together with a variable ERR_NR they serve to complete the
program with the previously given message without provoking an
actual error. To this end, the program should complete the
instruction GOTO 9999 and in this line to put the command:
9999 POKE 23621, L-256 * INT (L/256):
POKE 23622, INT (L/256):
POKE 23610, M-1: POKE 23623, K
In this case, the program will end
message:
M TEXT L: K.
ERR_NR # 5C3A (23610)
In this cell number appears 1 error has occurred and is
signaled in a message. The contents of this byte can be
valuable in write their own procedures for processing
errors in assembly, since immediately determine the cause of
termination of the program.
E_PPC # 5C49 (23625)
This variable (2 bytes) contains a number
current line and available for editing. Its modification will
cause the same effect that LIST N, but without a withdrawal
program screen. This makes it possible to select the keyboard
lines of code for modification, and then copying it to the
bottom of the screen and adjust without destroying the contents
of the screen.
MODE # 5C41 (23617)
This variable informs the editor of a cursor must be used in
this moment. The system uses the following values: 0-L, C or K,
1-E, 2-G. These numbers then used in various expressions
to assign a character code, which must be displayed as the
cursor. Programming in BASIC can only induce SPECTRUM,
to the nearest instructions INPUT instead
standard cursor L and C used
cursor to G or E. Where E is the mode
act only for the first input
symbol. E mode turned on POKE 23617, 1.
Interesting effects provides accommodation in this
other variable values (other than 1,
1, 2). POKE 23617, K for K = 3 ,..., 127 causes the cursor to
G, although the screen does not will be printed letter G, but
something else. POKE 23617, K for K = 0, 128 ,..., 255 the
regime of L or C, but with a conclusion on another screen
blinking cursor.
FLAGS # 5C6A (23658)
This variable determines what is required mode L or C. Is
responsible for this third bit FLAGS2. POKE 23658, 0 installs
regime L, and POKE 23658, 8 - Mode C. These changes get from
the keyboard using the CS / 2.
DF_SZ # 5C6B (23659)
This variable contains the number of rows
bottom of the screen reserved for
system messages and data entered from the
using INPUT. Initialized
2. Practically not used, although sometimes it can be used to
protect programs from the interrupt. Putting it 0
caused a breakdown of the system as soon as
it is necessary to output anything
the bottom of the screen (for example, messages
about pressing BREAK). However, it is not advantageous
weapons, because at the same time makes it impossible to use
instructions INPUT, CLS, and also can not ask a question
"SCROLL?".
OLDPPC # 5C6E (23662) and OCPPC # 5C70
(23664)
They contain the line number and team
line, which will be a transition in the case of issuing the
command CONTINUE. Modification of these variables in the
program allows to find an exact alternative to manual GO TO. In
some situations it is better than the modification NEWPPC and
NSPPC, etc. since the transition occurs only when meeting
CONTINUE, but not immediately after update
variables. These variables are automatically
corrected by the system whenever
termination of the program comes with a message different from
0 OK.
SEED # 5C76 (23670)
This framework of pseudo-random generator
numbers. Using SEED considered
when describing the function of RND.
FRAMES # 5C78 (23672)
Three bytes are the internal clock
ZX-SPECTRUM. They represent the number of PEEK
23672 +256 * PEEK 23673 +65536 * PEEK 23674 and
determine how many 1 / 50 seconds have elapsed since system
initialization. Maximum value is 2 ^ 1.24 = 16,777,215, which
meets 3 days 21 hours 12 minutes and 24.
3 seconds. The accuracy of the timer is
about 0. 01% or less than 9 seconds per day. This
Timer is activated at a time when
SPECTRUM serves external devices
(Printer, tape recorder, a speaker), as well as in
run time in machine code, which shut down or bypass control
for masked interrupts.
DATADD # 5C57 (23639)
This variable stores the address of an element in
the list of DATA, which will be read by another statement READ
(this is close to the truth but it is a good approximation of
the two cells). Memorizing their contents and using it
afterwards, you can get an accurate alternative command RESTORE
K.
SCR_CT # 5C8C (23692)
This byte determines the withdrawal
How many rows 1, the screen should appear the question
"SCROLL?". If you need to output is not interrupted, then at
least once on 255 lines must be derived in this box Record 255.
If we need to stop before the conclusion, it is necessary
set this variable to a value less
23.
6. 4 variables that serve Screen
TV
This group of variables is responsible for regular
interaction between the computer with a TV. One of them
controls the color, others define the place of displaying the
next sign or graphic symbol.
BORDCR # 5C48 (23624)
This variable contains the attributes that describe the
bottom of the screen, as well as frame color. Under normal
circumstances, ZX-SPECTRUM can not install the same color
for the background and the ink in the lower rows (you always
need to see them introduced symbols). Specifiers colors in the
list INPUT will be effective only for texts
printed at the bottom of the screen, but the color
ink for the input data is always set to 9 (white or black,
depending on the color of the background). A workaround is to
(Eg if the computer needs to read keyboard secret password)
using command POKE 23624, 128 + F +64 * B +8 * P + I and
assigning letters F, B, P, I parameters command FLASH, BRIGHT,
PAPER and INK, necessary to obtain the desired
effect. Those who are not convinced, we recommend
get this effect in another way:
BORDER 3: POKE 23624, 222: CLS.
ATTR_P # 5C8D (23693) and ATTR_T # 5C8F
(23695)
Both single-byte variables and store
attribute values FLASH, BRIGHT, PAPER and
INK. Letter P stands for the value set by constantly relevant
commands for the entire program, letter T - the current values
set by the same most teams in the Index
appropriate writing and portraying teams.
In the absence of color in the qualifiers
lists of writing instruction, variable
ATTR_P copied ATTR_T. Way of storing information about the
colors in a single byte is described in the chapter "Using the
memory." ATTR_P variable can be used in BASIC program for the
establishment of attributes of a single instruction POKE.
ATTR_T, in turn, may be important for programming in native
code, since commonly used procedures for the memory
ROM for setting the desired colors.
MASK_P # 5C8E (23694) and MASK_T # 5C90
(23696)
These byte variables are used to implement commands FLASH,
BRIGHT, PAPER, INK with parameter 8. The value of letters
P and T in the variable names the same thing
for both variables ATTR. Establishing a bit of these variables
in 1 means that the bit with the same number of corresponding
attribute byte should remain unchanged. Note that Team INK 8
establishes a 1 all 3 significant bits of the variable MASK_P.
Modifying this variable instruction POKE can, For example, set
to 1 only one least significant bit. Then it will be received
by the filter effect. Unchanged in the color of ink will remain
only the main blue component, while while others may be subject
to change. (Note that Numbering colors quite accidental. Color
blue - 1 red - 2, green - 3 is the main color of the mixture
which gives all the others. In the ZX-SPECTRUM role mixing does
add numbers corresponding colors.
COORDS # 5C7D (23677)
Two consecutive bytes of the variable containing the
coordinates (X, Y) points on the screen, which has completed
drawing last instruction PLOT, DRAW, CIRCLE.
Modifying this variable has the same
same effect as the PLOT OVER 1, K, N:
PLOT OVER 1, K, N, ie, the offset pointer screen without
drawing any point or lines.
SPOSN # 5C88 (23688)
This variable (2 bytes) contains the value 33-K, 24-M, where
K, M - coordinates last displayed on screen character.
Direct modification of these variables is difficult, since we
must simultaneously modify DF_CC.
DF_CC # 5C84 (23684)
This variable contains the byte address to
screen, which to begin displaying the next character
instruction PRINT. Modified only with Z_POSN. Much easier to
apply the command PRINT AT K, N.
SPONSL # 5C8A (23690) and DFCCL # 5C86
(23686)
Variables, similar S_POSN and DF_CC,
describing the bottom of the screen.
P_FLAG # 5C91 (23697)
Variable containing information about the print mode and
drawing on the screen. Describes the regimes established by
using the BASIC INVERSE, OVER, INK 9 and PAPER 9. Odd bits
correspond to the permanent assignment of these regimes, even
temporarily.
__________________________________
| | Temporary | permanent |
| | Bits | bits |
|-----------|----------|-----------|
| OVER 1 | 0 | 1 |
| INVERSE 1 | 2 | 3 |
| INK 9 | 4 | 5 |
| PAPER 9 | 6 | 7 |
|___________|__________|___________|
If you need to install permanent
OVER 1: INVERSE 1: INK 9: PAPER 9,
instead of 4 teams, only one POKE
23 697 2 8 32 128.
NOTE: The names of system variables
are not recognized by BASIC. They were introduced by the
authors of the operating system and taken into all the
literature related to ZX-SPECTRUM.
