Difference between revisions of "83Plus:OS:How BCALLs work"

Latest revision as of 23:06, 3 May 2013

Sorry, this article isn't very well written. It would be good if someone else could maybe look at the TI-OS code a bit and explain this better.

BCALLs are the way to call TI-OS ROM routines, and also the way that the branch table works in multi-page apps. A BCALL is the RST 28h instruction, followed by a 16-bit number which is used to identify the routine it is calling.

If the first bit (bit 15) of the number is set, this is what the OS does:

• It resets bit 15
• It adds 0x4000 to the number
• It maps page 1F/3F/7F/FF* into memory space 4000-7FFF

Otherwise, if bit 15 is reset and bit 14 is set, this is what the OS does:

• It resets bit 14
• It adds 0x4000 to the number
• It maps page 1B/3B/7B/05** into memory space 4000-7FFF

Otherwise, if bit 15 and 14 are both reset, it is treated as a multipage app branch, which is explained in the TI developer's guide.

Once the OS has mapped the appropriate page into 4000-7FFF, it uses the number as a pointer to a vector on this page. On pages 1B and 1F there are lists of vectors for the actual ROM routines. Each vector is 3 bytes - the first two bytes are a memory address and the third byte is the page that the routine is on.

*1F for the TI-83 Plus, 3F for the TI-84 Plus, 7F for the TI-83 Plus Silver Edition & the TI-84 Plus Silver Edition, and FF for the TI-84+CSE.
**1B for the TI-83 Plus, 3B for the TI-84 Plus, 7B for the TI-83 Plus Silver Edition & the TI-84 Plus Silver Edition, and 05 for the TI-84+CSE.

FloppusMaximus provided the following table on UnitedTI, having worked it out out of bordem:

                        T-states taken to perform a B_CALL to...
Platform                   App       Page 0   Other OS    Boot
--------------------------------------------------------------
TI-83+, OS 1.12            570         541       542       532
TI-83+, OS 1.18            879         819       820       810
TI-83+ SE, OS 1.18      875/880(*)     815       816       806
TI-84+, OS 2.41        1261/1248(**)  1045      1046      1036
TI-84+ SE, OS 2.41     1257/1244(**)  1041      1042      1032
(*) Slightly faster if the app is located in the first 32 pages of
    Flash.
(**) Slightly faster if interrupts are disabled.

See Also

• ROM Page 1B