The special USER commands U1 (or UA) and U2 (or UB) can be used to replace
the BLOCK-READ and the BLOCK-WRITE commands.
The format for U1 is:
“U1:”ch;dr;t;s
U1 forces the character count (buffer pointer) to 255 and reads an entire block
into memory. This allows complete access to all bytes in the block.
The format for U2 is:
“U2:”ch;dr;t;s
U2 writes a buffer to a block on the disk without changing the contents of
position 0 as B-W does. This is useful when a block is to be read in (with B-R)
and updated (B-P to the field and PRINT#), then written back to diskette with
U2.
Refer to the random access example in Chapter 6 for an application of the U1
and U2 commands.
Table 3 STANDARD JUMP TABLE
Alternate
User User
Designation Designation Function
U1 UA BLOCK-READ replacement
U2 UB BLOCK-WRITE replacement
U3 UC jump to $0500
U4 UD jump to $0503
U5 UE jump to $0506
U6 UF jump to $0509
U7 UG jump to $050C
U8 UH jump to $050F
U9 UI jump to $0065
U: UJ power up vector
U3 through U9 commands are user defined. The locations jumped to are located
in the buffer areas of RAM and routines may be written to reside there and
uploaded using the M-W command.
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