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Ohio Scientific zero page memory map
This memory map is from an Ohio Scientific C2-4P computer with 16K or memory and a BASIC-IN-ROM version 1.0, Rev 3.2. Most entries are due to Bruce Hoyt and Jim Butterfield. The Ohio Scientific used a 6502 CPU, so the first 256 bytes, or page zero of memory, was very special, as it could be accessed easier than any of the other memory in the system.
Zero Page
00 4C 74 A2 JMP to warm start. $BD11 earlier, cold start.
03 4C C3 A8 JMP to message printer. Registers A,Y contain the lo,hi address of the start of message. Message ends with a null
06 05 AE INVAR, USR get argument routine address
08 C1 AF OUTVAR, address of the USR return value routine
0A 4C 88 AE JMP to USR(X) routine
0D 00 number of nulls after a line feed, set by NULL command
Note! not the nulls after a carriage return
0E 00 line buffer pointer
0F 48 terminal width $48=72
10 38 input column limit
11 00 40 integer address
13 to 5A line buffer
5B 22 used by dec. to bin. routine, search character, etc.
5C 22 scan between quotes flag
5D -- line buffer pointer, number of subscripts
5E -- default DIM flag
5F FF type: $FF=string, $00=numeric
60 -- DATA scan flag, LIST quote flag, memory flag
61 00 subscript flag, FNx flag
62 -- $00=input, $98=read
63 -- comparison evaluation flag
64 00 CTRL-O flag. $80= means supress output
65 68 65 00 temporary string (descriptor stack) pointers
68 06 92 A1 stack of descriptors for temporary strings
6B -- -- -- stack of descriptors for temporary strings
6E -- -- -- stack of descriptors for temporary strings
71 92 A1 temporary variable pointer, also used by dec to bin
73 47 9B pointers, etc.
75 -- -- product staging area for multiplication
77 -- -- product staging area for multiplication
79 01 03 TXTTAB: address of start of the source program in RAM.
7B 03 03 VARTAB: address of start of the single variable table
7D 03 03 ARYTAB: address of start of the array variable table
7F 03 03 STREND: address of start of the empty basic memory
81 FF 3F FRETOP: address of start of the high string storage space
83 -- -- FRESPC: point to top of RAM memory. User can set to less than max RAM.
85 00 40 MEMSIZ: address + 1 of the end of basic memory
87 -- FF CURLIN: current line number
89 -- -- OLDLIN: line number at stop, end, or CTRL-C break
8B -- 00 program scan pointer, address of current line
8D -- -- line number of present DATA statement
8F 00 03 next address in DATA statements
91 -- -- address of the next value after comma in present DATA statement
93 -- -- last variable name
95 12 -- last variable value address
97 -- -- address of current variable, pointer for FOR/NEXT
99 -- -- -- work area, pointers, constant save, etc.
9C -- -- -- work area, pointers, constant save, etc.
9F -- 03 work area, pointers, constant save, etc.
A1 4C -- 00 JMP, a general purpose JMP
A4 -- -- -- misc. work area and storage
A7 -- FE 00 misc. work area and storage
AA -- -- -- pointer to current program line
AC to B0 first floating point accumulator. E,M,M,M,S
AC 06 92 AD and AE are printed in decimal by $B962
AE 68 FACHI, byte transferred by USR(X)
AF 00 FACLO, byte transferred by USR(X)
B0 20 sign of Acc. #1
B1 -- series evaluation constant pointer
B2 00 accumulator #1 high order (overflow) word
B3 to B7 second floating point accumulator. E,M,M,M,S
B8 92 sign comparison, acc #1 vs #2
B9 A1 acc. #1 low order (rounding) word
BA 98 A1 series pointer
BC to D3 routine copied from $BCEE. It is the start of a subroutine to go through a line character by character.
BC E6 C3 INC lo byte of address of character
BE D0 02 BNE
C0 E6 C4 INC hi byte if needed
C2 AD 00 03 LDA with a character of the line
C5 C9 3A CMP #$3A is it a colon?
C7 B0 0A BCS branch is yes, statement done
C9 C9 20 CMP #$20 is it a space?
CB F0 EF BEQ branch if yes, get another character
CD 38 SEC set carry
CE E9 30 SBC #$30
D0 38 SEC set carry
D1 E9 D0 SBC #$D0
D3 60 RTS end of subroutine, character in A
D4 80 4F random seed
D6 C7 52 random seed
D8 to FF unused by BASIC
FB -- monitor load flag
FC -- monitor data byte
FD -- monitor data byte
FE -- -- monitor current address
03 4C C3 A8 JMP to message printer. Registers A,Y contain the lo,hi address of the start of message. Message ends with a null
06 05 AE INVAR, USR get argument routine address
08 C1 AF OUTVAR, address of the USR return value routine
0A 4C 88 AE JMP to USR(X) routine
0D 00 number of nulls after a line feed, set by NULL command
Note! not the nulls after a carriage return
0E 00 line buffer pointer
0F 48 terminal width $48=72
10 38 input column limit
11 00 40 integer address
13 to 5A line buffer
5B 22 used by dec. to bin. routine, search character, etc.
5C 22 scan between quotes flag
5D -- line buffer pointer, number of subscripts
5E -- default DIM flag
5F FF type: $FF=string, $00=numeric
60 -- DATA scan flag, LIST quote flag, memory flag
61 00 subscript flag, FNx flag
62 -- $00=input, $98=read
63 -- comparison evaluation flag
64 00 CTRL-O flag. $80= means supress output
65 68 65 00 temporary string (descriptor stack) pointers
68 06 92 A1 stack of descriptors for temporary strings
6B -- -- -- stack of descriptors for temporary strings
6E -- -- -- stack of descriptors for temporary strings
71 92 A1 temporary variable pointer, also used by dec to bin
73 47 9B pointers, etc.
75 -- -- product staging area for multiplication
77 -- -- product staging area for multiplication
79 01 03 TXTTAB: address of start of the source program in RAM.
7B 03 03 VARTAB: address of start of the single variable table
7D 03 03 ARYTAB: address of start of the array variable table
7F 03 03 STREND: address of start of the empty basic memory
81 FF 3F FRETOP: address of start of the high string storage space
83 -- -- FRESPC: point to top of RAM memory. User can set to less than max RAM.
85 00 40 MEMSIZ: address + 1 of the end of basic memory
87 -- FF CURLIN: current line number
89 -- -- OLDLIN: line number at stop, end, or CTRL-C break
8B -- 00 program scan pointer, address of current line
8D -- -- line number of present DATA statement
8F 00 03 next address in DATA statements
91 -- -- address of the next value after comma in present DATA statement
93 -- -- last variable name
95 12 -- last variable value address
97 -- -- address of current variable, pointer for FOR/NEXT
99 -- -- -- work area, pointers, constant save, etc.
9C -- -- -- work area, pointers, constant save, etc.
9F -- 03 work area, pointers, constant save, etc.
A1 4C -- 00 JMP, a general purpose JMP
A4 -- -- -- misc. work area and storage
A7 -- FE 00 misc. work area and storage
AA -- -- -- pointer to current program line
AC to B0 first floating point accumulator. E,M,M,M,S
AC 06 92 AD and AE are printed in decimal by $B962
AE 68 FACHI, byte transferred by USR(X)
AF 00 FACLO, byte transferred by USR(X)
B0 20 sign of Acc. #1
B1 -- series evaluation constant pointer
B2 00 accumulator #1 high order (overflow) word
B3 to B7 second floating point accumulator. E,M,M,M,S
B8 92 sign comparison, acc #1 vs #2
B9 A1 acc. #1 low order (rounding) word
BA 98 A1 series pointer
BC to D3 routine copied from $BCEE. It is the start of a subroutine to go through a line character by character.
BC E6 C3 INC lo byte of address of character
BE D0 02 BNE
C0 E6 C4 INC hi byte if needed
C2 AD 00 03 LDA with a character of the line
C5 C9 3A CMP #$3A is it a colon?
C7 B0 0A BCS branch is yes, statement done
C9 C9 20 CMP #$20 is it a space?
CB F0 EF BEQ branch if yes, get another character
CD 38 SEC set carry
CE E9 30 SBC #$30
D0 38 SEC set carry
D1 E9 D0 SBC #$D0
D3 60 RTS end of subroutine, character in A
D4 80 4F random seed
D6 C7 52 random seed
D8 to FF unused by BASIC
FB -- monitor load flag
FC -- monitor data byte
FD -- monitor data byte
FE -- -- monitor current address
Page $0100
100 to 10C ASCII numerals built in this space
10D to 12F ???
130 NMI interrupt location
1C0 IRQ interrupt location (can be overwritten by BASIC)
133 to 1FF BASIC stack
10D to 12F ???
130 NMI interrupt location
1C0 IRQ interrupt location (can be overwritten by BASIC)
133 to 1FF BASIC stack
Page $0200
200 to 20E used to output to the screen and tape
200 cursor location, initialized to contents of $FFE0
201 save character to be printed
202 temporary
203 LOAD flag, $80 means load from tape
204 temporary
205 SAVE flag, $0 means not save mode
206 repeat rate for CRT routine
207 to 20E part of scroll routine
207 LDA $D700,Y
20A STA $D700,Y
20D INY
20E RTS
20F to 211 unused
212 00 CTRL/C flag, not 0 means ignore CTRL/C
213 0D 96 0D 0D used by keyboard routine
217 ?
218 to 221 used in 600 board machines as follows
218 input vector
21A output vector
21C CTRL/C vector
21E load vector
220 save vector
200 cursor location, initialized to contents of $FFE0
201 save character to be printed
202 temporary
203 LOAD flag, $80 means load from tape
204 temporary
205 SAVE flag, $0 means not save mode
206 repeat rate for CRT routine
207 to 20E part of scroll routine
207 LDA $D700,Y
20A STA $D700,Y
20D INY
20E RTS
20F to 211 unused
212 00 CTRL/C flag, not 0 means ignore CTRL/C
213 0D 96 0D 0D used by keyboard routine
217 ?
218 to 221 used in 600 board machines as follows
218 input vector
21A output vector
21C CTRL/C vector
21E load vector
220 save vector
Page $0300 onwards
This was the free RAM that BASIC used to store the program. It started at $0300 and went to the top of the free RAM memory, unless the user specified less memory during the first cold start.
