Zeus Assembler

SOFTSYNC, INC.

TIMEX/SINCLAIR 2068





ZEUS

Z80 ASSEMBLER











CONTENTS



INTRODUCTION

SECTION 1 LOADING AND RUNNING ZEUS

SECTION 2 WRITING MACHINE CODE WITH ZEUS

SECTION 3 ENTERING AND EDITING TEXT

3.1 General Format

3.2 Constants

3.3 Operators

3.4 Expressions

3.5 Assembler Directives

3.6 Screen Editor

SECTION 4 ZEUS ASSEMBLER COMMANDS

SECTION 5 THE MONITOR

APPENDIX 1 Command list

APPENDIX 2 Error report codes

APPENDIX 3 Table of reserved words

APPENDIX 4 ZEUS memory map

APPENDIX 5 Useful ZEUS routines

APPENDIX 6 Further facilities available





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.



The following terms are used throughout the rest of this manual and their

definitions are provided here for those unfamiliar with assembler

terminology.



SOURCE FILE: The text, including all assembly language mnemonics, labels

and comments is collectivel known as the source file.



OBJECT CODE: The object code is the machine code produced by ZEUS from

the source file.



SYMBOL: A symbol is a string of alphanumeric characters used to

represent a numerical value (either data or an address).

A label is 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.





SECTION 1

LOADING AND RUNNING ZEUS



If you wish to use the ZX printer with ZEUS then remember to connect it

up before plugging in your Spectrum.



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



2. Set the volume on your recorder to about 3/4 maximum and 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. ZEUS will auto-run on loading, displaying the copyright message and

flashing cursor.





SECTION 2

WRITING MACHINE CODE WITH ZEUS



This section is the 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 lenght 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 5100 IF BC<>0 THEN GOTO 5050

OR C

JR NZ,loop

RET 5110 RETURN



Firstly, 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 might use



LIST 1000



Here, the parameter 1000 causes the command to commence listing from line

1000.



To illustrate the use of parameters type:



I 100 100



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 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 value of the LIST command parameter is the first

line number.



To enter the first few lines of text:



1. Clear the screen (CAPSHIFTED '1').



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

text to be edited by altering it as it is displayed on the screen.



Firstly it is necessary to obtain a listing of the source file. Clear

the screen and enter 'L' (short for LIST). The LIST command takes up

three parameters of the form:



L x y z



where x=line number to start 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 position.

Remember that the modified line will only be inserted into the text on

pressing ENTER when the cursor is sitting on that line.



For example, to change line 30 to read:



0030 LD A,254



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

place the cursor in the same character square as the right had '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 desireable 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

defineable' TAB facility. The TAB stops may be set up from within the

Monitor. Type 'M' and 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 'L's in the listing at the top of the

screen. Now type a non-spcae 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. Firstly

enter 'I 60' 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 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. Firstly, you must tell ZEUS where to place the assembled code

by using the directive ORG.



Enter 5 ORG 40000



to assemble the machine code from memory location 40000.



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

execution 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 file is now 16 lines long.



ZEUS also features a renumber command of the form:



R x y z



See ZEUS Aseembler commands (Section 4) 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 accrodance with the

default values of the 'R' command.



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



0010 ORG 40000

0020 ENT

0030 LD HL,16384

0040 LD DE,16385

0050 LD A,255

0060 LD (HL),A

0070 LD BC,6143

0080 loop LD A,(HL)

0090 LD (DE),A

0100 INC HL

0110 INC DE

0120 DEC BC

0130 LD A,B

0140 OR C

0150 JR NZ,loop

0160 RET



Now you are ready to assemble the source file to produce the object code,

commonly code 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 have been 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 on 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 adviseable 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 byte

y=lengh 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:



i) Using the 'N' (short for NEW) command before you enter the program.

The parameter specifies the address from which the source will b

built, or



ii) Reloading an old source file toa different location to that from

which it was saved (see Section 4, 'T' command).



When you commenced entering lines of text, ZEUS automatically started the

source file from the default value 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 start the test routine. Enter 'X' (short for

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

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

the source code 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. Yet 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.





SECTION 3

ENTERING AND EDITING TEXT



3.1 General Format



ZEUS uses the ASCII character set. To enter assembly language instructions

you must firstly 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:



a) An optional label

b) An instruction

c) An optional comment



a) Optional label



The following rules govern the use of labels:



i) A label may contain upper and lower case letters and digits,



ii) A label must start with a letter,



iii) Labels can be up to 14 characters in length,



iv) 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 (eg. 'HELD' would be valid). For a list of reserved words see

appendix three.



v) 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.



b) 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.



c) Optional comment



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

separated from the instruction by a semicolon. Lines containing just

comments are allowed.



3.2 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





3.3 Operators



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

the use of logical operators.



The operators provided are:



+ addition

- subtraction

& logical AND

! logical OR



No operator priority is observed; expressions are evaluated strictly from

left to right.





3.4 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.





3.5 Assembler Directives



The following 'pseduo-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 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 nnnn 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, givent he following two statements at the

start of the 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



DEFS nnnn inserts nnnn spaces at the current assembly address



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 '/' delimeters 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





3.6 Screen Editor



ZEUS provides a full screen editor to facilitate modification of the source

file.



Firstly 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 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 hte inverse video key (CAPSHIFTED '4'). The character at the

edge of the screen is lost.



The character at the cursor positon may be deleted by using the

true video key (CAPSHIFTED '3').



Having altered a 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').



NB 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 5).





SECTION 4

ZEUS ASSEMBLER COMMANDS



A command consists of a command letter followed by any number of numerical

or string parameters.



Numerical parameters consists of either a decimal or hexadecimal constant.

Entry of numerical parameters overwrites the default parameters for that

command.



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

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

be skipped.



eg. L Lists the text from start to finish

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

L , 50 Lists the text from start to line 50



String parameters are represented by enclosing the string within

delimeters. Delimeters 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 5).



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 : the first new line number

y : the spacing between successive line numbers

z : the 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



i) the same address it was saved from, use:



LOAD "filename"CODE



ii) a different address, use:



LOAD "filename"CODE startaddress



Having loaded an old source file, re-enter ZEUS

(PRINT USR 57344) and enter the command



O startaddress



to retrieve the source file starting at 'startaddress'



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.





SECTION 5

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 hexadecimal

representations of the value x which may be of either form.



C x y z Copies 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



M x Enter modify mode starting at address x.

The modify command provides an extremely flexible and

hence powerful memory manipulation tool.

For example,

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:

a) 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.

b) Alter the contents of the address and subsequent

addresses simultaneously.

Type the new value of 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.

c) Alter the modification address.

Type '/' followed by the new address. On pressing

ENTER, modify will move to the new address.

d) Exit from modify mode

Type a full stop (period) and press ENTER.



Notes: i) 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.

ii) More complex formats can be entered. For example:

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 80

into address 6500 to 6502. Finally it prints 6503

followed by the 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 in 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 it 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 tabulate 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 1

Command List



i) Assembler



A x ASSEMBLE. Print x errors at a time.

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

F "string" x y z FIND 'string' between lines x and 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 to BASIC

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

with line x in increments of y.

S x Print SYMBOL table x lines at a time

T Display start address and length of the source file

X EXECUTE object code at last ENT statement





ii) Monitor



A x Print x in decimal and hex

C x y z COPY a block of memory from x to y of length z bytes

E x EXECUTE a machine code routine at address x

I x Print the value of I/O port x

K x y Set up 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 tab stops

T x y TABULATE memory from address x, y lines at a time

Z Return to assembler





iii) 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 at the cursor position

CAPSHIFTED '5', '6', '7' & '8' Cursor directional control keys

CAPSHIFTED '9' ('graphics') Clear the screen and home the cursor





APPENDIX 2

Error Report Codes



NUMBER MEANING

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 symbol

7 '(' symbol expected

8 Illegal mnemonic

9 Undefined label





APPENDIX 3

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 4

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 Defined by the user through the

(Object code) 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 insreases 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 occur 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 5

Useful ZEUS routines



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 zeroes



Notes: 1) All prints send characters to the printer as well as the VDU

if PRINTERFLAG is non-zero

2) PRINTCHAR allows the use of screen editing commands. For

example, printing byte #0F will clear the screen.



The list of editor control charcaters is as follows:



#04 Delete character under cursor

#05 Insert space at cursor position

#06 Move cursor position to next TAB position

#07 Clear the line containing the cursor

#08 Move cursor left

#09 Move cursor right

#0A Move cursor donw

#0B Move cursor up

#0C Move cursor left and blank character under cursor

#0D Move cursor to the beginning of the next line

Scroll if on bottom line

#0F Clear screen and cursor home





APPENDIX 6



Further machine code utilities from Crystal Computing:



ZX Spectrum Machine Code Monitor & 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 and Disassembler. You

may enter both utilities separately, but it id convenient to enter

ZEUS by using the Monitor command:



Goto E000



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

Monitor and Disassembler.