80-Bus News

ZAP

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 them anyway.