Zeus Assembler

USER MANUAL FOR

ZEUS ASSEMBLER





First published in 1983 by

Sinclair Research Limited



(c) Copyright Crystal Computing Ltd 1983





Contents



1 Introduction ................................ 5

2 Definitions ................................. 5

3 Loading and running ZEUS .................... 6

4 Writing machine code with ZEUS .............. 6

5 Entering and editing text ................... 12

5.1 General format .......................... 12

5.2 Constants ............................... 14

5.3 Operators ............................... 14

5.4 Expressions ............................. 14

5.5 Assembler directives .................... 15

5.6 Screen editor ........................... 16

6 ZEUS assembler commands ..................... 17

7 The monitor ................................. 20



Appendix one: Command list .................. 24

Appendix two: Error report codes ............ 25

Appendix three: Reserved words ................ 26

Appendix four: ZEUS memory map ............... 26

Appendix five: Useful ZEUS routines .......... 28

Appendix six: Further facilities ............ 29





1 Introduction



Welcome to ZEUS, the assembler-based machine code operating

system for the ZX Spectrum.

ZEUS provides the optimum method for writing Z80 machine

code on your computer and yet, as you will shortly discover, it is

even easier to use than BASIC!

The heart of ZEUS is a high speed two-pass assembler

designed to allow the convenient symbolic representation of your

machine code program: lines of assembly language mnemonics

(machine code instructions) are entered as simply as you would

enter a BASIC program. But here the similarity with other assemblers

ends, for the true power of ZEUS stems from the unique versatility

of its operating system. Just as the Sinclair ROM enables you to

write BASIC, so ZEUS provides all the facilities you need to produce

machine code programs quickly and efficiently.





2 Definitions



The following terms are used throughout this manual; we give their

definitions here for those unfamiliar with assembler terminology.



SOURCE FILE: The text, including all assembly language

mnemonics, labels and comments.

OBJECT CODE: The machine code produced by ZEUS from the

source file.

SYMBOL: A string of alphanumeric characters used to

represent a numerical value (either data or an

address).

LABEL: A special type of symbol, the value of which

corresponds to the address of the instruction

which immediately follows it.

DIRECTIVE: An assembler directive is not an actual Z80

instruction but represents an order given by the

programmer to the assembler which results in

the storing of values either into symbols or into

memory.





3 Loading and running Zeus



If you wish to use your ZX Printer with ZEUS, remember to connect

it before plugging in your Spectrum.



1. Rewind the cassette to the beginning of side A.

2. Set the volume on your recorder to about 3/4 maximum and the

tone to full treble.

3. Connect the EAR socket on your recorder to the EAR socket at

the rear of the computer.

4. Type LOAD "zeus" or simply LOAD "". Press ENTER and start

the tape.

5. Stop the tape when you see the copyright message and flashing

cursor.





4 Writing machine code with ZEUS



This section is a description of the entry of a short machine code

subroutine which, along with the program itself, has been written to

illustrate some of the features of ZEUS.

The program loads byte 255 into each address (memory

location) on the screen. The screen is 6144 bytes in length and

starts at address 16384. The outline of the program along with its

BASIC equivalent is as follows:



LD HL,16384 5000 LET HL=16384

LD DE,16385 5010 LET DE=16385

LD A,255 5020 LET A=255

LD (HL),A 5030 POKE HL,A

LD BC,6143 5040 LET BC=6143

loop LD A,(HL) 5050 LET A=PEEK HL

LD (DE),A 5060 POKE DE,A

INC HL 5070 LET HL=HL+1

INC DE 5080 LET DE=DE+1

DEC BC 5090 LET BC=BC-1

LD A,B

OR C

JR NZ,loop 5100 IF BC<>0 THEN GOTO 5050

RET 5110 RETURN



First clear the screen by pressing CAPSHIFTED '9' (equivalent

to entering 'graphics' mode in Sinclair BASIC).

Entering lines of text is very similar to writing a BASIC program.

Each line must be prefixed by a number which may take any value

from 0 to 65534 inclusive. It is important to remember that, providing

the line the cursor is on begins with a valid line number, pressing

ENTER will insert that line at the correct position in the source file so

replacing any previously existing line which had the same number.

The command to tell ZEUS to provide line numbers automatically

is of the form:

I x y

where x=line number from which to start

y=increment between successive line numbers

x and y are called 'parameters' because they specify the particular

values which the command will use when it executes. For example,

in a long BASIC program you may use

LIST 1000

Here, the parameter 1000 causes the command to start listing from

line 1000.

To illustrate the use of parameters type:

I100 100

and press ENTER several times. To exit from auto-line number

mode simply delete the current line number (eg. with CLEARLINE:

CAPSHIFTED '1') and press ENTER.

Clear the screen (CAPSHIFTED '9'), type 'I' and press ENTER

several times. As you have not specified particular values for the

parameters x and y, ZEUS has used the 'default' values which are

x = 10 and y= 10. To take an example from BASIC, the default of

the LIST command parameter is the first line number.

To enter the first few lines of text:



1. Clear the screen (CAPSHIFTED '9')

2. Type 'I' followed by ENTER (once only)

3. Type in the following lines of text, pressing ENTER after each to

insert the line in the source file and to produce the next line

number:

LD HL,16384

LD DE,16385

LD A,255

LD (HL),A

LD BC,6143

If you enter a character incorrectly use ERASE (CAPSHIFTED

'0') and re-type.

4. Exit from auto-line number mode

(CAPSHIFTED '1' followed by ENTER).



You may now use the full screen editor which, as the name

suggests, allows the text to be edited by altering it as it is displayed

on the screen.

First obtain a listing of the source file. Clear the screen and

enter 'L' (short for LIST). The LIST command takes up to three

parameters of the form:

L x y z

where x=line number to list from

y=line number to list up to

z=number of lines to list before pausing



To alter a particular character in the text use the cursor

control keys (CAPSHIFTED '5', '6', '7' & '8') to move the cursor to

the desired point in the listing and simply type in the replacement

character. The cursor will move automatically to the next character

position. Remember that the modified line will be inserted into the

text on pressing ENTER only when the cursor is sitting on that

line.

For example, to change line 30 to read:

00030 LD A,254

first obtain a listing of the text and using the cursor control keys

place the cursor in the same character square as the right hand

'5' digit. Type '4' and press ENTER, CLEARSCREEN and LIST.

You may now change the number back to 255 either by the same

method or by entering

30 LD A,255

Before proceeding, CLEARSCREEN and LIST.

The next part of the program involves a loop. It is often

desirable to indent loops in order to make a program more

readable. To speed the process of entering sections of indented

text, ZEUS provides a 'user definable' TAB facility. The TAB

stops may be set up from within the Monitor. Type 'M' and press

ENTER. The word 'Monitor' appears to remind you of the mode

you are in. Next type 'S' and move the cursor along until it is lined

up with the column of Ls in the listing at the top of the screen.

Now type a non-space character (eg. 1) followed by, say, 5

spaces, a further 1 and ENTER. CAPSHIFTED '2' will now move

the cursor to the stops you have just defined.

Return to ZEUS (enter "Z"). Now you are ready to add the

loop. First enter "I60" to invoke auto-line numbering from line 60.

Type in the word 'loop' (which you may wish to put in lower case

characters by using the CAPSHIFT key). Use CAPSHIFTED '2' to

put the cursor at the next TAB stop. Now type:

LD A,(HL)

and press ENTER. Enter the following lines remembering to

indent by using CAPSHIFTED '2':

LD (DE),A

INC HL

INC DE

DEC BC

LD A,B

OR C

JR NZ,loop

Note that each reference to a particular label must be

identical, character for character, to that label. Hence if you have

used lower case in line 60 then you must do the same in line 130.



Enter the last line without the indent, ie:

00140 RET

Exit from auto-line number mode (CAPSHIFTED '1' followed by

ENTER).

Two additional lines are necessary before you can assemble

and test the routine. First, you must tell ZEUS where to place the

assembled code by using the directive ORG.

Enter 5 ORG 30000

to assemble the machine code from memory location 30000.

Second, you must set an entry point, ie. the point from which

the execute command ('X') will run the code. Enter:

6 ENT

followed by CLEARSCREEN and LIST.

You will notice that it is necessary to press ENTER twice to list

the complete source file. This is because the default value for the z

parameter (number of lines to list before pausing) is 14 and the

source listing is now 16 lines long.

ZEUS also features a renumber command of the form:

R x y z

See ZEUS Assembler commands (Section 6) for definition of the

parameters.

To renumber the source file, enter 'R'. Now list the first 5 lines

by typing:

L,,5

and pressing ENTER once only. Note that the use of a comma to

replace a parameter causes the command to use the default value

of that parameter. You can see that the source has been renumbered

in accordance with the default values of the 'R' command.



Check carefully that the source file is identical to the following:

00010 ORG 30000

00020 ENT

00030 LD HL,16384

00040 LD DE,16385

00050 LD A,255

00060 LD (HL),A

00070 LD BC,6143

00080 loop LD A, (HL)

00090 LD (DE) ,A

00100 INC HL

00110 INC DE

00120 DEC BC

00130 LD A,B

00140 OR C

00150 JRNZ,loop

00160 RET



Now you are ready to assemble the source file to produce the

object code, commonly called the machine code. Type 'A' (short for

'assemble') and press ENTER.

If all is well the cursor will simply move below the 'A'. If you

have made a mistake, an error report will be issued followed by the

offending line. If the error is not apparent, consult the appendix on

error report codes.

At this stage the easiest mistake to make is to omit a space

from within an instruction. This will produce error 0 (incomplete

statement). You may find it worthwhile to purposely introduce an

error by, for example, removing the space in line 30.

(CLEARSCREEN, LIST, place the cursor in the space between 'D'

and 'H', DELETE (CAPSHIFTED '3'), ENTER, CLEARSCREEN,

ASSEMBLE).

You should see:

Error 0

0030 LDHL,16384

To correct the error, move the cursor to the 'H' and use

INSERT (CAPSHIFTED '4') to add the space, ASSEMBLE,

CLEARSCREEN and LIST.



It is advisable to save the source file on tape prior to execution.

Loading and saving is accomplished by using the BASIC commands

and treating the source file as a block of data bytes.

The BASIC command to save a block of bytes is of the form:

SAVE "filename" CODE x,y

where x=address of the first data byte

y=length of data

The values of the parameters x and y are given by the 'T'

command. Enter 'T and you should see

start of source=32768

length =00144

Return to BASIC by entering 'Q' (short for QUIT) and use

SAVE "source 1" CODE 32768,144

to save the source to tape. Check your recording with

VERIFY "" CODE

To return to ZEUS enter

PRINT USR 57344

Before you can list the source you must enter 'O' to tell ZEUS

to retrieve the old source file. The 'O' command can also take a

parameter, that being the start address of the file it is to retrieve.

You may create a source file at a different address by either:

- Using the 'N' (short for NEW) command before you enter the

program. The parameter specifies the address from which the

source will be built, or

- Reloading an old source file to a different location from that from

which it was saved (see Section 6, 'T' Command).

When you began to enter lines of text, ZEUS automatically

started the source from the default of the 'N' command, ie. address

32768 (or 8000 hex.)

If you have a ZX Printer attached then you may make a 'hard

copy' of the source file. To turn the printer 'on', enter 'P1'. Now enter

L,,16

When the printer is on, all output sent to the VDU is also sent to

the printer. To turn the printer off enter 'P0'. You may interrupt the

printer by using the BASIC BREAK key. Return to ZEUS in the

normal way.

You are now ready to test the routine. Enter 'X' (short for

EXECUTE). If all is well, the screen (apart from the border) will fill

with the current ink colour (ie. white). If this does not happen, list the

source and check it carefully against the listing above. If the

computer crashes, simply reload ZEUS, QUIT, and load the source

file using LOAD "" CODE. Remember to use the 'O' command to

retrieve the old source file.

In this section you will have gained an appreciation of the way

ZEUS can be used to produce a simple subroutine. There are many

additional features at your disposal to aid the programming of more

complex routines. The next section provides a comprehensive

description of these and also consolidates the features already

mentioned.





5 Entering and editing text

5.1 General format



ZEUS uses the ASCII character set. To enter assembly language

instructions you must first type in a line number and follow it with the

required line of text. The line number must lie in the range 0-65534

inclusive.

The text must consist of one or more statements separated by

colons. Each statement comprises:



1. An optional label,

2. An instruction,

3. An optional comment.



5.1.1 Optional label



The following rules govern the use of labels:

- A label may contain upper and lower case letters and digits.

- A label must start with a letter.

- A label can be up to 14 characters in length.

- A label must not be identical to a reserved word (eg. using

'LD' as a label is not permitted). However, a label may contain

reserved words (eq, 'HELD' would be valid). For a list of

reserved words see Appendix three.

- A label must be separated from an instruction by a space.

Note that every reference to a particular label must be identical,

character for character, to that label.



5.1.2 Instruction



An instruction may be any of the standard Zilog Z80 instructions or it

may be an assembler directive (see below). An extra facility is

provided by ZEUS for referring to the parity/overflow flag. When

using ZEUS you may refer to 'jump overflow' instead of 'jump parity

even'. Thus:

JP PE,... may be written JP V,...

and JP PO,... may be written JP NV,...

The two versions are completely interchangeable but their use can

help to make a routine more readily comprehensible.



5.1.3 Optional comment



A comment may be appended to the end of any instruction. It must

be separated from the instruction by a semicolon. Lines containing

comments only are also allowed.



5.2 Constants



Constants may be expressed either in decimal or hexadecimal form

as follows:

Decimal: 1 99 234 4096

Hexadecimal: #A #FE #6843 #5C00

Literals can be expressed as follows:

"A "7 "? "# "=

For example, to load the literal 'C' into the accumulator, use:

LD A,"C

Note that in Z80 convention, parentheses denote an address

eg. LD A,(200) will load the accumulator with the contents of

memory location 200 (decimal).

Finally, there is also a system constant '$' which is set to the

current assembly address, eg:

DJNZ $ is equivalent to loop DJNZ loop



5.3 Operators



In order to further facilitate the writing of symbolic programs, ZEUS

allows the use of logical operators:

+ addition

- subtraction

& logical AND

! logical OR

No operator priority is observed: expressions are evaluated

strictly from left to right.



5.4 Expressions



Wherever a constant is required in an instruction, an expression

may be used in its place. Expressions are built from labels and/or

constants separated by operators, eg:

LD A,(ADDRESS+offset)

LD HL,START-3

IN A,(PORT & #FF)

LABEL !#FF00

Note that expressions will be computed by the assembler to

determine the actual memory location which will be inserted into the

object code. Expressions are computed at assembly time, not at

program execution time.



5.5 Assembler directives



The following 'pseudo-operations' either provide parameters for the

operation of ZEUS or instruct the assembler to store values either

as symbols or directly into memory:



ORG nnnn Short for ORIGIN. This directive instructs ZEUS

to assemble the block of machine code (as

translated from the source listing after the ORG

statement) from address nnnn (provided that the

current DISP value is zero - see below).

Multiple ORGs within the same source file are

allowed. Each ORG statement will redirect the

address from which subsequent code is

assembled.

DISP nnn Short for DISPLACEMENT. A DISP instruction

alters the place from which subsequent code is

generated even though the code so produced is

assembled to run at the address specified by the

current ORG parameter. It is sometimes

convenient to be able to generate code at a

different location from the one at which it is

ultimately intended to run (for example if the

latter is occupied at present). The monitor may

be used to relocate the code to the ORG

address.



For example, given the following two statements

at the start of a source file:

ORG 30000

DISP 10000

the source following would be assembled from

40000 but would only normally run at 30000.



ENT Set an entry point. The 'X' command executes

the assembled code from the last ENT directive

in the source file.



EQU Short for EQUATE or EQUALS. A label (symbol)

may have a value assigned to it using a statement

of the form:

label EQU value



DEFB nn,nn,.. Inserts bytes nn at the current assembly address.



DEFW nnnn,nnnn, Inserts words (addresses) nnnn at the current

assembly address.



DEFM/string/ The text enclosed in the '/' delimiters will be

inserted at the current assembly address.



As with all assembly language instructions, directives may be

prefixed by a label, eg:

DATA1 DEFW 40000,3456,6789

OFFSETS DEFB 3,2,14,20,9,2

USRADDR ENT



5.6 Screen editor



ZEUS provides a full screen editor to facilitate modification of the

source file.

First list the text by using the 'L' command. The cursor may

now be moved to any point in the listing by using the cursor control

keys (CAPSHIFTED '5', '6', '7', '8').

To replace the character at the current cursor position simply

enter the new character. The cursor will move automatically to the

next character position.

Space may be inserted into a line at the current cursor position

by using the inverse video key (CAPSHIFTED '4'). The character at

the edge of the screen is lost.

The character at the cursor position may be deleted by using

the true video key (CAPSHIFTED '3').

Having altered the line, pressing ENTER will insert the new line in

the correct place in the text.



Other commands are:



CLEARSCREEN: The entire screen may be cleared and the cursor

placed at the top left hand corner by using the

'graphics' key (CAPSHIFTED '9')



CLEARLINE: The current line may be cleared and the cursor

placed at the start of the line using the 'edit' key

(CAPSHIFTED '1')

N.B. This command does not remove the current

line from the text but simply from the screen.



TAB: The 'Caps lock' key (CAPSHIFTED '2') moves

the cursor to the next tab stop on the current line.

The tab stops may be set up from within the

Monitor (See Section 7).





6. ZEUS Assembler Commands

A command consists of a command letter followed by any number

of numerical or string parameters.

Numerical parameters consist of either a decimal or a

hexadecimal constant. Entry of numerical parameters overwrites the

default parameters for that command.

If it is desired to alter a later default parameter without upsetting

earlier ones, this can be achieved by entering a comma for every

parameter to be skipped, eg:

L List the text from start to finish

L 10 50 List the text from line 10 to line 50

L , 50 List the text from the start to line 50

String parameters are represented by enclosing the string

within delimiters. Delimiters can be any character except ',' or '#' or

any digit, eg:

F "LDIR" finds all occurrences of the string LDIR

F /LDIR/ ditto



Commands



A x Assemble the source file and display error

messages. After x errors have been encountered

and displayed, assembly will stop. To continue

assembly, press ENTER.

DEFAULT: x=14



D x y Delete all lines between line x and line y

inclusively. If no parameters are given, no action

will be taken.



F"string" x y z This command searches the source file for all

occurrences of the given string. The search

commences at line x and finishes at line y. Any

lines containing the string are displayed and a

pause initiated after z lines have been displayed.

DEFAULT: x=lowest line number

y=highest line number

z=14

"string"=" "



I x y After entry of this command, ZEUS will

automatically generate line numbers, starting

with line number x. Any text entered will be

inserted in the correct place in the source file and

another line number generated. Parameter y is

the spacing between successive line numbers.

Deleting the line number and pressing ENTER

will halt auto line numbering.

DEFAULT: x=10 y=10



L x y z List the source file from line x to line y. Parameter

z gives the number of lines to be listed before

pausing.

DEFAULT: x=start y=finish z=14



M Enter the Monitor (See Section 7).



N x Create a new, empty source file at address x.

NB: care must be taken when using this command

as placing a source file at certain locations may

cause a crash.

DEFAULT: x=32768 (#8000)



O x This command causes the 'old' source file at

address x to become the current source; it is

usually used to retrieve a source file which has

been loaded to address x.

DEFAULT: x=32768 (#8000)



P x Printer operation. x=1 turns the printer on. All

output to VDU is also sent to the ZX Printer.

Pressing BREAK during printer operation causes

a return to BASIC. Re-enter ZEUS as normal

(the printer is automatically switched off on

re-entry). x=0 turns the printer off.

DEFAULT: x=0



Q QUIT. Exit to BASIC. To return to ZEUS, enter

PRINT USR 57344



R x y z Renumber the source file. The parameters are

as follows:

x=first new line number

y=spacing between successive new line numbers

z=line to start re-numbering from.

DEFAULT: x=10 y=10 z=lowest current line number



S x Print the symbol table. x gives the number of

symbols/labels and their associated hexadecimal

values listed before pausing.

DEFAULT: x=15



T Display the start address and length of the

current text file in bytes. The file may be saved by

returning to BASIC ('Q') and using:

SAVE "filename" CODE start,length

To reload an old source file to

- the same address as it was saved from, use:

LOAD "filename" CODE

- a different address, use:

LOAD "filename",start_address

Having loaded an old source file, re-enter ZEUS

(PRINT USR 57344) and enter the command

O start_address

to retrieve the source file at 'start_address'



X Execute the machine code produced by the last

assembly. Execution begins at the ENT statement

nearest to the end of the source file.

If no ENT statement existed, an error message is

issued and no execution occurs.





7 The monitor

The co-resident Monitor allows you to directly inspect and manipulate

memory and the I/O ports and also set up several options within ZEUS.



MONITOR COMMANDS



A x This command displays both the decimal and the

hexadecimal representation of the value x which

may be of either form, eg:

A 59 prints HEX=#3B DECIMAL=59

A #2A prints HEX=#2A DECIMAL=42



C x y z Copy a block of memory. This command takes

three parameters, as follows:

x=start address of block to be copied from

y=start address of block to be copied to

z=the number of bytes to copy



E x Execute machine code from address x.



I x Print the value of I/O port x.



K x y This command allows the border, paper and ink

colours to be altered. x is the byte which is used

to fill the attributes table: it is made up of four

fields:

Bits 0-2 control the ink colour

Bits 3-5 control the paper colour

Bit 6 is set for 'bright'

Bit 7 is set for 'flash'

y is the required border colour

eg: K F 1: white ink on blue background with

blue border

K 38 7: black ink on white background with

white border.



M x Enter modify mode starting at address x.

The modify command provides an extremely

flexible and hence powerful memory manipulation

tool, eg:

M 6000

will print:

6000 XX (note the position of the cursor)

XX is the present contents of address 6000. You

may now proceed in four ways:

1. Alter the contents of the address

Type the new value over the old one and press

ENTER.

Modify prints the next address, 6001, along with

its present contents.

2. Alter the contents of the address and

subsequent addresses simultaneously.

Type the new value for the present address

followed by the values to be written in the

subsequent addresses. On pressing ENTER,

modify will print the address and value of the byte

after the last one modified.

3. Alter the modification address.

Type '/' followed by the new address. On

pressing ENTER, modify will move to the new

address.

4. Exit from modify mode.

Type a full stop and press ENTER.

Notes: Characters can be entered by the use of quotes,

eg:

after entering M 6000 overwrite the present value

with "C. The ASCII value of the character 'C' is

now at address 6000.

More complex formats can be entered, eg:

6000 43 41 40 "A "C "D.

enters the six values into 6000 to 6005 and

leaves modify mode.

6000 43 41 40 / 6500 83 81 80

enters bytes 43 41 40 into 6000 to 6002. It then

moves the modify address to 6500 and enters 83

81 and 80 into addresses 6500 to 6502. Finally it

prints 6503 followed by contents of that address.

The above sequences can be combined with the

'enter modify mode' command ('M'), eg:

M 6000 FF 42.

enters modify mode, puts FF into 6000 and 42

into 6001 and leaves modify mode.



O x y Output the value y to I/O port X





S Set TAB stops. Any non-space character on the

same line as the 'S' will define a TAB stop

(provided the line is entered). CAPSHIFTED '2'

advances the cursor between the stops, eg:

S 1 1

sets TAB stops at the columns occupied by the

'1' character.



T x y Tabulate memory from address x. The contents

of memory are displayed eight bytes at a time in

hexadecimal form. Each line is prefixed by the

address of the first byte on that line. After y lines

have been displayed, tabulation will halt. To

continue, press ENTER; any other key will return

you to normal (command) mode.



Note that is is possible to modify memory by the

following method: tabulate the required area of

memory as described above and after returning

to command mode, enter modify mode ('M').

Using the cursor control keys, move the cursor

up into the tabulated listing. After altering all the

required values on a given line press ENTER to

modify the actual memory locations.



Exit from modify mode as before by typing a full

stop and pressing ENTER.



Z Return control to the assembler.



Note that all constants entered in the Monitor must be hexadecimal

(with the exception of the 'A' command which, like the assembler,

defaults to decimal); prefix hex values with a '#' symbol.





Appendix one

Command List



1. Assembler



A x ASSEMBLE. Print x errors at a time.

D x y DELETE from line x to line y inclusive.

F"string" x y z FIND 'string' between line x and line y; print

z occurrences at a time.

I x y INSERT (auto line number) starting at line x with

increments of y.

L x y z LIST from line x to line y in steps of z lines.

M Enter MONITOR.

N x Create a NEW source file at address x.

O x Recover OLD source file at address x.

P x PRINTER control: x=0 for 'off'; x=1 for 'on'.

Q QUIT. Return to BASIC.

R x y z RENUMBER the source file from line z to

commence with line x and increments of y.

S x Print the SYMBOL TABLE x lines at a time.

T Display the start address and length of the

source file.

X EXECUTE object code at last ENT statement.



2. Monitor



A x Print x in decimal and hex.

C x y z COPY a block memory from address x to address y

for a length z bytes.

E x EXECUTE a machine code routine from address x.

I x Print the value of I/O port x.

K x y Set up the paper, ink and border colours.

M x Enter MODIFY mode at address x.

O x y OUTPUT byte y to I/O port x.

S SET up the TAB stops.

T x y TABULATE memory from address x, y lines

at a time.

Z Return to the assembler.



3. Screen editor



CAPSHIFTED '1' ('edit')

Clear the line containing the cursor

CAPSHIFTED '2' ('caps lock')

Move the cursor to the next TAB stop

CAPSHIFTED '3' ('true video')

Delete the character under the cursor

CAPSHIFTED '4' ('inv. video')

Insert a space character at the cursor position

CAPSHIFTED '5', '6', '7' & '8'

Cursor directional control keys

CAPSHIFTED '9' ('graphics')

Clear the screen and home the cursor





Appendix two

Error report codes



0 Illegal character or incomplete statement

1 Label too long

2 ')' symbol expected

3 Truncation error or jump out of range

4 ',' expected

5 Context error

6 Redefined label

7 '('symbol expected

8 Illegal mnemonic

9 Undefined label





Appendix three

Reserved words



The following is a list of reserved words, ie. those which must not be

used as labels, although any label may contain them.



A ADC ADD AF' AF AND B BC BIT

C CALL CCF CP CPD CPDR CPI CPIR CPL

D DAA DE DEC DEFB DEFM DEFS DEFW DI

DISP DJNZ E EI ENT EQU EX EXX H

HALT HL I IM IN INC IND INDR INI

INIR IX IY JP JR L LD LDD LDDR

LDI LDIR M NC NEG NOP NV NZ OR

ORG OTDR OTIR OUT OUTD OUTI P PE PO

POP PUSH RES RET RETI RETN RL RLA RLC

RLCA RLD RR RRA RRC RRCA RRD RST SBC

SCF SET SLA SP SRA SRL SUB V XOR

Z





Appendix four

ZEUS memory map



The following areas of memory are used by ZEUS:



A) ZEUS machine code and workspace

57344-65279 (#E000-#FF00)

B) Source file

Initially starts at 32768 (#8000) but

may be altered with the 'N' command.

C) Assembled code (Object code)

Defined by the user through the

directives ORG & DISP

D) Symbol table

This is the table which contains the alphanumeric

symbols and labels defined in the source file

along with their associated values. It starts at

57343 and increases in size downwards at a rate

of sixteen bytes per label used.



Note: if the symbol table (produced on assembly) becomes

close enough to the source to be in danger of overwriting it, the

message:

"Out of symbol space"

is issued. Similarly if, when entering text, the source file becomes

too close to ZEUS the message:

"Out of memory"

is produced.

If either of these occurs, the source file must be moved down in

memory. This can be done using the Monitor 'COPY' command.

However, great care should be exercised to ensure that, if necessary,

RAMTOP is redefined using the BASIC command CLEAR before

copying down.





Appendix five

Useful Zeus routines



You may wish to call some of the following ZEUS subroutines from

your own programs:



NAME ADDRESS FUNCTION

INPUTCHAR #F652 Wait for a key to be pressed; result in A

PRINTCHAR #F503 Print char in A. May also be a

control character (see below). If

printer flag is non zero will also

output to printer

PRINTDECIMAL #E5A3 Write HL in decimal followed by a space

PRINTHEXBYTE #F2DF Write A in hex followed by a space

PRINTHEXWORD #E571 Write HL in hex followed by a space

PRINTSTRING #E4E3 Write the string following the call;

string must be terminated by byte

zero.

INPUTLINE #F6E2 Enter screen editor, put line which

cursor is on in buffer

BUFFER #FE00 32 character buffer terminated by

a byte zero

PRINTERFLAG #F4CB Non-zero to enable printer

PRINTDECNZ #E59E Print decimal number in HL

without leading zeros



Notes: All prints send characters to the printer as well as VDU if

PRINTERFLAG is non-zero. PRINTCHAR also allows the

use of screen editing commands. For example, printing byte

#0F will clear the screen.



The list of editor control characters is as follows:



#04 Delete character under cursor

#05 Insert space at cursor position

#06 Move cursor position to next tab position

#07 Clear line containing cursor

#08 Move cursor left

#09 Move cursor right

#0A Move cursor down

#0B Move cursor up

#0C Move cursor left and blank character under cursor

#0D Move cursor to beginning of next line

Scroll if on bottom line

#0F Clear screen and cursor home





Appendix six

Further machine code facilities from Crystal Computing:



ZX Spectrum Machine Code Monitor and Disassembler Package



This program provides additional facilities to aid the inspection,

analysis and debugging of your routines; with this and ZEUS

co-resident in memory your Spectrum becomes a truly

comprehensive Z80 programming unit.

NB. When you intend to use both programs at once, load ZEUS

in FIRST, then QUIT and load in the 16K Monitor/Disassembler.

You may enter both utilities separately, but it is convenient to enter

ZEUS by using the Monitor command:

Goto E000

The use of the ZEUS command QUIT will then return control to

the Monitor and Disassembler.