Advanced programmers may find the following table useful as it shows the contents of important block zero RAM locations used by BASIC or the Cassette Operating System. Locations marked as miscellaneous working space tend to have multiple uses. For example, B0 to DB are used by COS for different purposes at different times, and 16 to 59 are usually but not always used as 4-byte integer arithmetic registers.
Note that many of these explanations are not formally supported by Acorn, so they may change in future ROMs. If this happens, we at Timedata would be interested to hear about it.
| Address | |
| 00 | Error number |
| 01-02 | Current Line number (binary) |
| 03 | Pointer to character being investigated in statement being interpreted. |
| 04 | Temp. for X |
| 05-06 | Address of start of statement being interpreted |
| 07 | Print Column counter |
| 08-0C | Random number seed |
| 0D 0E | Top (binary) |
| 10 11 | Address of BASIC error handler, normally C9E7, |
| 12 | Text space pointer; normally 82 or 89 |
| 16- 59 | Misc. BASIC working space |
| 5A 5B | X ordinate for point plot routine |
| 5C 5D | Y ordinate for point plot routine |
| 5E | Data for point plot routine. 1=set, 2=invert, else clear |
| 5F 60 | Point plot routine workspace |
| 61 6F | Misc. BASIC working space |
| 70 7F | FP. BASIC working space |
| 80 AF | Free |
| B0 DB | Misc. COS working space |
| DC | Tape read/write checksum |
| DD | Fload flag; bit 7 = 1 to ignore block numbers |
| DE DF | VDU RAM address of
start of line containing the cursor DE should be 00, 20, 40 ... E0, DF should be 80 or 81 |
| E0 | Cursor position in VDU line; 00 to 3F (or is that 1F?) |
| E1 | 0=cursor off, 1 = cursor on |
| E2 E5 | Misc. Workspace |
| E6 | Number of lines
remaining (1-16) if VDU in page mode, bit 7 = 1 if not in page mode. |
| E7 | Bits 5,6,= 1 for shift lock, else 0 |
| E8 E9 | Misc. Workspace |
| EA | 'Nomon' if not zero |
| EB FD | Misc. Workspace for COS |
| FE | Character not sent to printer (normally 0A = Line Feed) |
| 100 13F | Input line buffer |
| 140 17F | String processing area |
| 180 1FF | Stack |
| 200 201 | Ptr. to NMI routine; not set by COS or BASIC |
| 202 203 | Ptr. to BRK routine; normally C9D8 |
| 204 205 | Ptr. to IRQ routine; normally A000 (Utility ROM) |
| 206 207 | Ptr. to Command Line Interpreter, normally F8EF |
| 208 209 | Ptr. to Write Char. routine, normally FE52 |
| 20A 20B | Ptr. to Read Char. routine, normally FE94 |
| 20C 20D | Ptr. to Load File routine, normally FE6E |
| 20E 20E | Ptr. to Save File routine, normally FAE5 |
| 210 211 | Ptr. to RDRVEC routine, normally |
| 212 213 | Ptr. to STRVEC routine, normally |
| 214 215 | Ptr. to GetTapeByte routine, normally |
| 216 217 | Ptr. to PutTapeByte routine, normally |
| 218 219 | Ptr. to PrintTapeMessage routine, normally |
| 21A 21B | Ptr. to Shut file routine, normally |
| 21C 23F | Free |
| 240 2EA | Misc. Workspace for BASIC |
| 2EB 320 | Pointers to arrays: 2EB,306 = AA; 2EC,307 = BB etc. |
| 321 | Integer Variable |
| 322 33C | Low byte of variables A-Z |
| 33E 357 | 2nd byte of variables A-Z |
| 359 371 | 3rd byte of variables A-Z |
| 373 38C | High byte of variables A-Z |
| 38D 3C0 | Labels; 38D, 38E is
RAM address of label a, 38F, 340 is RAM address of label b, etc. |
| 3C1 3FD | Misc. Workspace for BASIC |
| 3FE 3FF | Ptr. to point plot
routine F6E2 for mode 0, F73B for mode 1, F754 for mode 2, F76D for mode 3, F7AA for mode 4, |