Changing RAMTOP Without “NEW”

Editor’s note: Although this pearl of programming poetry is not a direct Pro/File enhancement, it is never the less, a utility with many useful applications for people who like to cut and hack into Pro/File or any other program. Now for Richard’s wisdom:

Those of us who use 64K RAM on our ZX81/TS1000 must poke RAMTOP and execute NEW before loading large programs. This can also be done by a short machine code routine added to the high-speed loader programs like ZXLR-8 described in previous UPDATES.

This routine can be placed in a 35 character REMarks statement following any existing REMs in the loader (e.g. 2 REM for Z-XLR&8). The following list is poked into the 35 locations beginning with the first REMark character–for ZXLR-8 that is location 18833.

18833  42
18834  4
18835  64
18836  167
18837  237
18838  114
18839  229
18840  193
18841  237
18842  91
18843  4
18844  64
18845  42
18846  50
18847  64
18848  34
18849  4
18850  64
18851  237
18852  66
18853  249
18854  235
18855  237
18856  66
18857  237
18858  176
18859  27
18860  27
18861  27
18862  27
18863  237
18864  83
18865  2
18866  64
18867  201

The following BASIC routine can be used to POKE the REM:

7000 FOR I=0 T 34
7010 INPUT J
7020 POKE (18833+I),J
7030 NEXT I
7040 STOP

The routine reads the new RAMTOP value from the random number seed, which must be preset using RAND 65535. It computes the current length of the machine stack, copies the stack to the new end of memory, sets the new value into RAMTOP, sets ERR_SP, and points the stack pointer (SP register) to the new stack.

The Z80 instructions are:

            ; MOVE MACHINE STACK
            ; NEW RAMTOP IN RND SEED
            ; COPY STACK TO NEW TOP
            ; SET STACK POINTER TO 
            ; NEW STACK
            ; SET ERR-SP TO NEW END
            ; OF STACK
2A0440      LD HL,(4004)    ;OLD RAMTOP
A7          AND A           ;CLEAR CARRY
ED72        SBC HL,SP       ;STACK SIZE
E5          PUSH HL         ;MOVE TO COUNTR
C1          POP BC
ED5B0440    LD DE,(4004)    ;OLD RAMTOP
2A3240      LD HL,(4032)    ;NEW RAMTOP
220440      LD (4004),HL    ;SET RAMTOP
ED42        SBC HL,BC       ;NEW SP
F9          LD SP,HL        ;SET NEW SP
EB          EX DE,HL        ;SWAP PTRS
ED42        SBC HL,BC       ;OLD SP
EDB0        LDIR            ;COPY BOTTOM-UP
1B          DEC DE          ;NEW RAMTOP-1
1B          DEC DE          ;ADJUST TO ERRSP
1B          DEC DE
18          DEC DE
ED530240    LD (4002),DE    ;SET ERRSP
C9          RET

To include this stack mover in ZXLR-8, delete statements 9080 and 9082, and add the following to both expand the usable memory and automatically start the loader:

9100 REM MOVE MACHINE STACK 64K
9110 RAND 65535
9120 RAND USR 18833
9200 REM MOVE Z-XLR8
9210 RAND USR 18141
9220 REM EXECULTE ZXLR8
9230 RAND USR L

This routine can also be used (alone or with a loader) where the BASIC memory must be reduced to protect an area at the top of a 16K RAM. Just precede the RAND USR statement by RAND and the new RAMTOP value you want to have.