The BBC and Master Computer Public Domain Library
802C BASIC ROM service startup
Submitted by Steve Fewell
Description:
Firstly, A & Y are pushed to the 6502 stack (to save their value)
Then the value in A is checked and the following operations are carried out depending
on its value (listed are the service type numbers, the definition of the service number
and the execution address for the service call):
&09 - *HELP instruction expansion (issued by MOS when user enters *HELP instruction) --> &804C
&04 - Unrecognised command (issued by MOS for unrecognised *-command) --> &8070
&02 - Relative Private workspace claim (issued by MOS after a [BREAK]) --> &8040
&27 - A Reset has occurred (issed by MOS on hard reset) --> &8040 (same as &02, above)
If A contains none of the above values then [&806A] Y & A are pulled from the 6502 stack
and X is set to the RAM copy of the currently selected paged ROM (from zero page memory location &F4),
which is the ROM number of this BASIC ROM. Then the service call is exited (RTS).
Following are descriptions of these 3 service call routines
&8040 Write the BASIC ROM number
This routine services service calls &02 (issued after a [BREAK]) and &27 (issued after a hard reset - power-on
or [CTRL]+[BREAK]).
OSBYTE call &BB is issued to write the BASIC ROM number. It is called with:
X = contents of zero page location (&F4) - i.e. the currently selected ROM
Y = #&00 - the command for Writing the BASIC ROM number value.
So the MOS is told that the BASIC ROM number is that of the currently active ROM.
Next [&806A] is called to pull Y and A from the 6502 stack and to set X to the RAM copy of the currently
selected paged ROM (from zero page memory location &F4 - which should be the ROM number for this BASIC ROM)
and then exit the service call (RTS).
&804C
This routine services service call &09 which is issued during a *HELP command.
The next byte pointed to by the address in locations &F2-&F3 plus the offset in Y register is read.
This byte is the first byte after the '*HELP' part of the issued command.
If this byte is not a carriage-return (ASCII 13) then BASIC ignores the *HELP command as BASIC does
not provide help on any keywords, so [&806A] is called to pull Y and A from the 6502 stack and to set X to the
currently selected paged ROM (from zero page memory location &F4 - which should be the ROM number for this
BASIC ROM) and then exit the service call (RTS).
Otherwise, the *HELP command did not contain any keywords (parameters), so BASIC will just list its name and
version number as follows:
* Call OSNEWL (&FFE7) to start a new output line on the display.
* Read the BASIC ROM Name and Version number from location &8009 [&7F13 + &F6] to &8012 [&7F13 + &FF].
This is the text: "BASIC" + ASCII 0 + "4r32". Each character is output using the OSASCI (&FFE3) routine,
except that the ASCII 0 character is replaced by a space.
OSNEWL (&FFE7) is called again to start a new output line.
Next [&8067] &80D8 is called to clear Bit 6 of memory location &F4 (the RAM copy of the current paged ROM).
Lastly, [&806A] pull Y and A from the 6502 stack, set X to the currently selected paged ROM (from zero page
memory location &F4 - which should be the ROM number for this BASIC ROM) and then exit the service call (RTS).
&8070
This routine services service call &04 which is issued when an unrecognised *-command is entered.
The first letter of the unrecognised command (pointed to by the address at &F2-&F3 + the offset in the Y register)
is read. This letter is ANDed with #&DF (1101 1111) to convert the letter to uppercase (if it was lowercase).
If this letter is "H" (the first letter of HIBASIC) then:
* Bit 6 of the RAM copy of the current paged ROM (address &F4) is set.
* Read the next letter of the unrecognised command.
* If the letter is "." then jump to &80A8.
* If the letter (once converted to uppercase -> AND#&DF) is "I" then jump to &808F.
* Otherwise if not "." or "I", then the command is not "HIBASIC", so:
- Call [&80D8] to clear Bit 6 of memory location &F4 (the RAM copy of the current paged ROM).
- Decrement Y by 2 (so it points again to the first letter of the unrecognised command).
- Call [&80D8] to clear Bit 6 of memory location &F4 (the RAM copy of the current paged ROM).
[&808F] This location is reached both if the letters "HI" are found, or if they were not found (and Y points again
at the start of the unrecognised command). The difference is that if "HI" was found that bit 6 of location &F4 will
be set. As I guess the MOS doesn't use this bit of the location, BASIC uses it to determine wither HIBASIC (via TUBE)
is active or standard BASIC (without TUBE relocation).
Next, the unrecognised command (starting at location Y) will be checked against the string "BASIC". The ROM name in
the ROM header (locations &8009 [&7F0E + &FB] to &800D [&7F0E + &FF]) is used to compare against the unrecognised
command (after converting the unrecognised command letter to uppercase -> AND#&DF).
If a "." is found then a match is assumed and address &80A8 is jumped to.
If any letter fails to match against the corresponding letter of the ROM name "BASIC", then the unrecognised
command is unrecognised by BASIC (i.e. it is not "HIBASIC" or "BASIC"), so:
[&8067] &80D8 is called to clear Bit 6 of memory location &F4 (the RAM copy of the current paged ROM).
then: pull Y and A from the 6502 stack, set X to the currently selected paged ROM (from zero page memory location
&F4 - which should be the ROM number for this BASIC ROM) and then exit the service call (RTS).
If the unrecognised command matches against "BASIC", then the next byte of the unrecognised command is checked.
If this byte is greater than or equal to "!" [ASCII 33] then the unrecognised command doesn't match "HIBASIC" or
"BASIC", as it doesn't terminate with a space or a carriage return [ASCII 13] after the "BASIC" string, so the
service call returns without a match as follows:
[&8067] &80D8 is called to clear Bit 6 of memory location &F4 (the RAM copy of the current paged ROM).
then: pull Y and A from the 6502 stack, set X to the currently selected paged ROM (from zero page memory location
&F4 - which should be the ROM number for this BASIC ROM) and then exit the service call (RTS).
[&80A8] A match is found - the unrecognised command is either "HIBASIC" or "BASIC"
Bit 6 (the bit corresponding to the overflow flag) of location &F4 is checked to determin whether the "HIBASIC"
or "BASIC" command was issued.
If Bit 6 is clear then BASIC will be started as follows:
* [&80CC] Place the ROM number from location &F4 into X (after disregarding bit 6 -> EOR#&40)
* Jump to OSBYTE &8E (*FX 142) to enter the BASIC language ROM (the ROM Number in X).
* Exit the service call (via the jump statement above).
Otherwise, Bit 6 is set and the "HIBASIC" command was issued.
So:
Call &BF66 to detect if the Tube is present.
If Tube is present then jump to &80CC to:
* [&80CC] Place the ROM number from location &F4 into X (after disregarding bit 6 -> EOR#&40)
* Jump to OSBYTE &8E (*FX 142) to enter the BASIC language ROM (the ROM Number in X).
(Note: OSBYTE &8E automatically handles the relocation of the ROM Code to the High &B800 address).
* Exit the service call (via the jump statement above).
Otherwise, Tube is not present so:
[&80B1] &80D8 is called to clear Bit 6 of memory location &F4 (the RAM copy of the current paged ROM).
Copy the following error BRK statement from locations &80C2-&80CB to locations &0100-&010A:
BRK
EQUB 0
EQUS "No TUBE"
BRK
Then jump to the code at &0100 to cause the MOS to display the Error "No TUBE".
Note: I'm not sure why BASIC needs to copy the error code instead of just executing it straight from the ROM!
Changes from BASIC IV:
The addition of a service address is completely new functionality that is not present in BASIC IV.
Additionally, BASIC IV did not have a TUBE version (e.g. HIBASIC) that specifically ran accross the TUBE.
Disassembly for the BASIC ROM service Startup routine
|
802C |
H |
72 |
48 |
PHA |
|
802D |
|
170 |
AA |
TAX |
|
802E |
|
152 |
98 |
TYA |
|
802F |
H |
72 |
48 |
PHA |
|
8030 |
|
224 009 |
E0 09 |
CPX#&09 |
|
8032 |
|
240 024 |
F0 18 |
BEQ 24 --> &804C |
|
8034 |
|
224 004 |
E0 04 |
CPX#&04 |
|
8036 |
8 |
240 056 |
F0 38 |
BEQ 56 --> &8070 |
|
8038 |
|
224 002 |
E0 02 |
CPX#&02 |
|
803A |
|
240 004 |
F0 04 |
BEQ 4 --> &8040 |
|
803C |
' |
224 039 |
E0 27 |
CPX#&27 |
|
803E |
* |
208 042 |
D0 2A |
BNE 42 --> &806A |
|
|
169 187 |
A9 BB |
LDA#&BB |
|
|
8042 |
|
166 244 |
A6 F4 |
LDX &F4 |
|
8044 |
|
160 000 |
A0 00 |
LDY#&00 |
|
8046 |
|
032 244 255 |
20 F4 FF |
JSR &FFF4 OSBYTE |
|
8049 |
Lj |
076 106 128 |
4C 6A 80 |
JMP &806A |
|
|
177 242 |
B1 F2 |
"LDA (&F2),Y" |
|
|
804E |
|
201 013 |
C9 0D |
CMP#&0D |
|
8050 |
|
208 024 |
D0 18 |
BNE 24 --> &806A |
|
8052 |
|
032 231 255 |
20 E7 FF |
JSR &FFE7 OSNEWL |
|
8055 |
|
162 246 |
A2 F6 |
LDX#&F6 |
|
8057 |
|
189 019 127 |
BD 13 7F |
"LDA &7F13,X" |
|
805A |
|
208 002 |
D0 02 |
BNE 2 --> &805E |
|
805C |
|
169 032 |
A9 20 |
LDA#&20 |
|
805E |
|
032 227 255 |
20 E3 FF |
JSR &FFE3 OSASCI (Write Character) |
|
8061 |
|
232 |
E8 |
INX |
|
8062 |
|
208 243 |
D0 F3 |
BNE -13 --> &8057 |
|
8064 |
|
032 231 255 |
20 E7 FF |
JSR &FFE7 OSNEWL |
|
8067 |
|
032 216 128 |
20 D8 80 |
JSR &80D8 |
|
h |
104 |
68 |
PLA |
|
|
806B |
|
168 |
A8 |
TAY |
|
806C |
|
166 244 |
A6 F4 |
LDX &F4 |
|
806E |
h |
104 |
68 |
PLA |
|
806F |
` |
96 |
60 |
RTS |
|
|
177 242 |
B1 F2 |
"LDA (&F2),Y" |
|
|
8072 |
) |
041 223 |
29 DF |
AND#&DF |
|
8074 |
H |
201 072 |
C9 48 |
CMP#&48 |
|
8076 |
|
208 023 |
D0 17 |
BNE 23 --> &808F |
|
8078 |
|
200 |
C8 |
INY |
|
8079 |
@ |
169 064 |
A9 40 |
LDA#&40 |
|
807B |
|
004 244 |
04 F4 |
TSB &F4 |
|
807D |
|
177 242 |
B1 F2 |
"LDA (&F2),Y" |
|
807F |
|
200 |
C8 |
INY |
|
8080 |
. |
201 046 |
C9 2E |
CMP#&2E |
|
8082 |
$ |
240 036 |
F0 24 |
BEQ 36 --> &80A8 |
|
8084 |
) |
041 223 |
29 DF |
AND#&DF |
|
8086 |
I |
201 073 |
C9 49 |
CMP#&49 |
|
8088 |
|
240 005 |
F0 05 |
BEQ 5 --> &808F |
|
808A |
|
032 216 128 |
20 D8 80 |
JSR &80D8 |
|
808D |
|
136 |
88 |
DEY |
|
808E |
|
136 |
88 |
DEY |
|
808F |
|
162 251 |
A2 FB |
LDX#&FB |
|
8091 |
|
177 242 |
B1 F2 |
"LDA (&F2),Y" |
|
8093 |
. |
201 046 |
C9 2E |
CMP#&2E |
|
8095 |
|
240 017 |
F0 11 |
BEQ 17 --> &80A8 |
|
8097 |
) |
041 223 |
29 DF |
AND#&DF |
|
8099 |
|
221 014 127 |
DD 0E 7F |
"CMP &7F0E,X" |
|
809C |
|
208 201 |
D0 C9 |
BNE -55 --> &8067 |
|
809E |
|
200 |
C8 |
INY |
|
809F |
|
232 |
E8 |
INX |
|
80A0 |
|
208 239 |
D0 EF |
BNE -17 --> &8091 |
|
80A2 |
|
177 242 |
B1 F2 |
"LDA (&F2),Y" |
|
80A4 |
! |
201 033 |
C9 21 |
CMP#&21 |
|
80A6 |
|
176 191 |
B0 BF |
BCS -65 --> &8067 |
|
80A8 |
$ |
036 244 |
24 F4 |
BIT &F4 |
|
80AA |
P |
080 032 |
50 20 |
BVC 32 --> &80CC |
|
80AC |
f |
032 102 191 |
20 66 BF |
JSR &BF66 Detect if Tube is present |
|
80AF |
|
208 027 |
D0 1B |
BNE 27 --> &80CC |
|
80B1 |
|
032 216 128 |
20 D8 80 |
JSR &80D8 |
|
80B4 |
|
162 010 |
A2 0A |
LDX#&0A |
|
80B6 |
|
189 194 128 |
BD C2 80 |
"LDA &80C2,X" |
|
80B9 |
|
157 000 001 |
9D 00 01 |
"STA &0100,X" |
|
80BC |
|
202 |
CA |
DEX |
|
80BD |
|
016 247 |
10 F7 |
BPL -9 --> &80B6 |
|
80BF |
L |
076 000 001 |
4C 00 01 |
JMP &0100 |
|
80C2 |
|
0 |
0 |
BRK |
|
80C3 |
|
0 |
0 |
BRK |
|
80C4 |
No TUBE |
078 111 032 084 085 066 069 |
4E 6F 20 54 55 42 45 |
EQUS "No TUBE" |
|
80CB |
E |
000 |
00 |
BRK |
|
80CC |
|
165 244 |
A5 F4 |
LDA &F4 |
|
80CE |
I@ |
073 064 |
49 40 |
EOR#&40 |
|
80D0 |
|
170 |
AA |
TAX |
|
80D1 |
|
169 142 |
A9 8E |
LDA#&8E |
|
80D3 |
|
160 000 |
A0 00 |
LDY#&00 |
|
80D5 |
L |
076 244 255 |
4C F4 FF |
JMP &FFF4 OSBYTE |
|
@ |
169 064 |
A9 40 |
LDA#&40 |
|
|
80DA |
|
020 244 |
14 F4 |
TRB &F4 |
|
80DC |
` |
96 |
60 |
RTS |
