Readers who program in machine code on the Spectrum may be interested in the following tip. When the USR command is used, both the zero and carry flags are reset to zero and the BC register pair holds the start address of the code to be executed.
The BC register value has an obvious use, that of relocating machine code. It is easy to prove that BC holds the address called by using POKE 25000,201 (i.e. RET) and then PRINT USR 25000. This will print 25000, that is, whatever is in BC is printed.
The reset carry flag can also be useful since it allows the machine code to distinguish between a call to the first byte or a call to the second byte of the program. For example:
LABELOLD
SCF
LABELNEW NOP
JR C,STARTOLD
STARTNEW
etc
RET
STARTOLD
etc
RET
What will happen is that a call made to the first byte of the above program causes the machine code from STARTOLD to execute, since the carry flag has been set with SCF. When LABELNEW is called, the carry flag is zero on entry and does not become set by the NOP, hence JR,C is ignored and the code from STARTNEW is executed.
This provides a way of cutting down on the number of addresses that need to be remembered. The second routine simply runs from the first address plus one.
15th September 1984
