I ordered my copy of ZAP from an old copy of 80-BUS NEWS where the price
quoted was #15. When it arrived, I was pleasantly surprised to find a refund
of #8.50 since the price had since been reduced to #6.50. It comes on a good
quality BASF tape and the manual is just 11 A4 pages stapled together. ZAP
takes up about 7K and also compresses the source code, so a minimum of 16K RAM
is sufficient. On cold starting you are greeted with a cheery message and a
report on the amount of free memory.
The editing facilities are the worst feature of this assembler and are
similar to what you find on most BASICs of a few years vintage. In particular
there is no pause control on listing to the screen, so you can only list in
blocks of 14 lines. Also, there is no string search/change command which,
together with the poor listing facilities, makes editing rather tedious. Once
you have suceeded in getting the required line on the screen it can be edited
using the NAS-SYS screen-editing commands. All commands must be entered in
full (usually four letters). This is far less convenient than single key
entry. Filenames are not used when saving the source on tape.
There are a couple of useful commands however, such as OBEY “c” where c
can be any NAS-SYS command, so you can use NAS-SYS commands without having to
leave the assembler. DUMP will write the assembled object code to tape and if
you put a RET instruction at the end of the program, RUN will execute the
program and return control to ZAP when it is finished.
ZAP also compresses the source by using 1-byte codes for mnemonics,
labels and macros, and by removing all unnecessary spaces. There is no space
between the line number and the label and only one space is left between label
and mnemonic, and mnemonic and comment. This makes it impossible to lay out
the source in neat columns for readability, since the resultant listing is
always a scruffy mess!
Conditional assembly is possible by enclosing the source within an IF
(expression)… FI statement. If the expression is evaluated to true (non-zero)
then the code within the block is assembled, otherwise it is ignored and
assembly continues from the line following the FI. Macros are also supported –
a macro is just a group of instructions which is given a suitable name. When
the macro name appears in the source, the assembler inserts the machine code
which makes up the macro. A macro would normally be used where a small group
of instructions are used many times, but where a subroutine would not be
appropriate. For example a macro to push registers at the start of each
subroutine. Parameters may be passed to the macro so that the same macro may
operate on different data, which may be registers, labels, expressions, etc.
In the ZAP assembly listing the macro appears in full with all the mnemonics,
so that it is still readable by anyone not familiar with macros. Labels and
macros may be listed at any time but if you have deleted any from the source
during programming, they still appear in the tables and it is imposible to get
rid of them! Assembly is aborted at the first error, however the error
messages given are reasonably full and clear.
Multistatement lines may be used with mnemonics separated by colons.
The NAS-SYS restart mnemonics are supported (although BRKPT and RDEL are
incorrectly represented by BREAK and KDEL respectively). In addition, “for
convenience” as the manual puts it, there are alternative mnemonics for some
of the more common instructions. e.g. CLA (clear acc) for XOR A, JSR (jump to
subroutine) for CALL. I really don’t see the point of including alternatives
since their use will only lead to lack of standardisation and the Zilog
mnemonics are quite logical and clear enough. Another unusual, but more
interesting feature is the inclusion of the Z80 unknown opcodes, or most of