8.4. Copying Data
in memory
Length: 33
Number of variables: 3
Checksum: 4022
Purpose: This program copies the memory from one
place to another.
Variables:
Name: start
Length: 2
Cell: 23296
Comment: The source address to copy.
Name: destination
Length: 2
Cell: 23298
Comment: address, which is copied.
Name: length
Length: 2
Cell: 23300
Comment: The length of the block,
to be copied.
Calling the program:
RANDOMIZE USR address
Error Check: No
Comment: this subroutine
can be used to create an "animation" using the following method:
- Creation of the first screen of information;
- Copy the screen above RAMTOP;
- Repeat for the other screens;
- Copy of screens in the opposite direction one by one in
quick succession.
LISTING machine code
Assembly label number to VVODA147.
LD HL, (23296) 42 0 91
LD DE, (23298) 237 91 February 1991
LD BC, (23300) 235 75 April 1991
LD A, B 120
OR C 177
RET Z 200
AND A 167
SBC HL, DE 237 82
RET Z 200
ADD HL, DE 25
JR C, COPY 56 3
LDIR 237,176
RET 201
COPY EX DE, HL 235
ADD HL, BC 9
EX DE, HL 235
ADD HL, BC 9
DEC HL 1943
DEC DE 1927
LDDR 237,184
RET 201
2
How it works:
In a couple of registers HL loaded address of the first byte
of memory for copying, DE loads the address to copy
memory, and a BC loaded the number of bytes to copy.
If BC = 0 or HL = DE, then the routine returns to BASIC. If
HL is greater than DE, part of the memory
copied, using the instructions LDIR and the program returns to
BASIC.
If DE is greater than the HL, then
to both pairs of registers is added to BC-1 and the memory is
copied using the instructions LDDR, then the program returns to
BASIC.
