INMC News

  

Summer 1979, Issue 3











Page 11 of 26











Firstly, you’ve got to remember that ZEAP is only about 2¾K long and that is pretty short as assemblers come. ZEAP was designed to run in a NASCOM with the minimum (8K) expansion possible, and because of this a few short cuts were taken. The short cuts all concern what are known as ‘assembler directives’ which are instructions to the assembler (not program instructions to the Z80) to do certain things with certain bytes of the program. For instance a ‘defined byte directive’ (DEFB), now this directive instructs the assembler to put the byte you have defined into the program. So DEFB £1A means put 1A into the program (the £ sign means the byte is expressed in hexadecimal). The assembler will assemble it like this; 0C50 is the address by the way 0010 is the line number:

0C50 1A      0010       DEFB £1A

Now for the short cut bit – suppose you have a string of defined bytes, followed by an instruction, the assembler would assemble them like this:–

0C50  1A          0010         DEFB £1A, £2C, £44, £5F
0C54  3E1F        0020         LD A, £1F

Notice that ZEAP only displayed the first defined byte, but the addresses worked out right. This is because ZEAP assembled them, it just does not display them. Notice also the second instruction, there is no space between the 3E and the 1F. Remember, if you were typing that into a NASCOM, you would have to put a space between the two.

The same point about defined bytes applies to defined words (DEFW) and defined messages (DEFM) thus;

0C50  1A          0010        DEFB £1A, £2C, £44, £5F
0C54  350C        0020        DEFW £0C35, £0035, £0124
0C5A  54          0030        DEFM /THATS THE LOT/
0C67  3E1F        0040        LD A, £1F
0004              0050        DEFS 4
0C6D  00          0060        NOP

Notice that with a defined space (DEFS) ZEAP simply put 0004 in the address field, and advanced the address counter by four.

So the basic message is, when loading programs assembled on ZEAP, watch the address counter.

Of course, lets not create a wrong impression, these short cuts in the display only occur on the monitor screen or on a printer, if you were using ZEAP yourself, these deficiences need not cause problems, because you would ultimately assemble the program to memory or tape, in which case these bytes would be correctly assembled even though they are not displayed.

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This is an OCR’d version of the scanned page and likely contains recognition errors.











Page 11 of 26