GitLab

Detail:
  This change gets rid of the following switches from the source (picking appropriate code paths for a 32bit HAL build):
  * FixCallBacks
  * UseProcessTransfer
  * CanLiveOnROMCard
  * BleedinDaveBell
  * NewStyleEcfs
  * DoVdu23_0_12
  * LCDPowerCtrl
  * HostVdu
  * Print
  * EmulatorSupport
  * TubeInfo
  * AddTubeBashers
  * TubeChar, TubeString, TubeDumpNoStack, TubeNewlNoStack macros
  * FIQDebug
  * VCOstartfix
  * AssemblingArthur (n.b. still defined for safety with anything in Hdr: which uses it, but not used explicitly by the kernel)
  * MouseBufferFix
  * LCDInvert
  * LCDSupport
  * DoInitialiseMode
  * Interruptible32bitModes
  * MouseBufferManager
  * StrongARM (new CacheCleanerHack and InterruptDelay switches added to hdr/Options to cover some functionality that StrongARM previously covered)
  * SAcleanflushbroken
  * StrongARM_POST
  * IrqsInClaimRelease
  * CheckProtectionLink
  * GSWorkspaceInKernelBuffers
  * EarlierReentrancyInDAShrink
  * LongCommandLines
  * ECC
  * NoSPSRcorruption
  * RMTidyDoesNowt
  * RogerEXEY
  * StorkPowerSave
  * DebugForcedReset
  * AssembleKEYV
  * AssemblePointerV
  * ProcessorVectors
  * Keyboard_Type
  Assorted old files have also been deleted.
Admin:
  Identical binary to previous revision for IOMD & Raspberry Pi builds


Version 5.51. Tagged as 'Kernel-5_51'

Detail:
  This set of changes tackles two main issues:
  * Before mapping out a cacheable page or making it uncacheable, the OS performs a cache clean+invalidate op. However this leaves a small window where data may be fetched back into the cache, either accidentally (dodgy interrupt handler) or via agressive prefetch (as allowed for by the architecture). This rogue data can then result in coherency issues once the pages are mapped out or made uncacheable a short time later.
    The fix for this is to make the page uncacheable before performing the cache maintenance (although this isn't ideal, as prior to ARMv7 it's implementation defined whether address-based cache maintenance ops affect uncacheable pages or not - and on ARM11 it seems that they don't, so for that CPU we currently force a full cache clean instead)
  * Modern ARMs generally ignore unexpected cache hits, so there's an interrupt hole in the current OS_Memory 0 "make temporarily uncacheable" implementation where the cache is being flushed after the page has been made uncacheable (consider the case of a page that's being used by an interrupt handler, but the page is being made uncacheable so it can also be used by DMA). As well as affecting ARMv7+ devices this was found to affect XScale (and ARM11, although untested for this issue, would have presumably suffered from the "can't clean uncacheable pages" limitation)
    The fix for this is to disable IRQs around the uncache sequence - however FIQs are currently not being dealt with, so there's still a potential issue there.
  File changes:
  - Docs/HAL/ARMop_API, hdr/KernelWS, hdr/OSMisc - Add new Cache_CleanInvalidateRange ARMop
  - s/ARM600, s/VMSAv6 - BangCam updated to make the page uncacheable prior to flushing the cache. Add GetTempUncache macro to help with calculating the page flags required for making pages uncacheable. Fix abort in OS_MMUControl on Raspberry Pi - MCR-based ISB was resetting ZeroPage pointer to 0
  - s/ARMops - Cache_CleanInvalidateRange implementations. PL310 MMU_ChangingEntry/MMU_ChangingEntries refactored to rely on Cache_CleanInvalidateRange_PL310, which should be a more optimal implementation of the cache cleaning code that was previously in MMU_ChangingEntry_PL310.
  - s/ChangeDyn - Rename FastCDA_UpFront to FastCDA_Bulk, since the cache maintenance is no longer performed upfront. CheckCacheabilityR0ByMinusR2 now becomes RemoveCacheabilityR0ByMinusR2. PMP LogOp implementation refactored quite a bit to perform cache/TLB maintenance after making page table changes instead of before. One flaw with this new implementation is that mapping out large areas of cacheable pages will result in multiple full cache cleans while the old implementation would have (generally) only performed one - a two-pass approach over the page list would be needed to solve this.
  - s/GetAll - Change file ordering so GetTempUncache macro is available earlier
  - s/HAL - ROM decompression changed to do full MMU_Changing instead of MMU_ChangingEntries, to make sure earlier cached data is truly gone from the cache. ClearPhysRAM changed to make page uncacheable before flushing cache.
  - s/MemInfo - OS_Memory 0 interrupt hole fix
  - s/AMBControl/memmap - AMB_movepagesout_L2PT now split into cacheable+non-cacheable variants. Sparse map out operation now does two passes through the page list so that they can all be made uncacheable prior to the cache flush + map out.
Admin:
  Tested on StrongARM, XScale, ARM11, Cortex-A7, Cortex-A9, Cortex-A15, Cortex-A53
  Appears to fix the major issues plaguing SATA on IGEPv5


Version 5.35, 4.79.2.306. Tagged as 'Kernel-5_35-4_79_2_306'

For disc based applications (ie. those that don't know the architecture at build time, like a ROM would) we have OS_PlatformFeatures to provide an abstract way of seeing when new chunks of instructions get added. Back at ARMv6 ARM deprecated SWP, but currently we have no way of knowing that at runtime without grubbing round the coprocessor registers.
Add 3 new flags
* One to say LDR/STREX is (not) available
* One to say that SWP/SWPB is (not) available
* One to say that CLREX and LDR/STREX[B|H|D] is (not) available
shame it took a few goes for ARM to bring in these variants, requiring 3 flags not 1.

Also:
Condition the exception on vector read code on No32bitCode, rather than just having it permanently disabled.
Improve the HAL device docs.

Tested on a StrongARM Risc PC, Model B Pi, and Titanium.

Version 5.35, 4.79.2.303. Tagged as 'Kernel-5_35-4_79_2_303'

Detail:
  s/ARMops - Change VMSAv6 cache policy mapping so that default NCB policy maps to Normal, non-cacheable memory rather than Device memory. This allows unaligned accesses to work, and also delivers some major performance improvements for some activities (write performance seems about 2x better than Device)
  Docs/HAL/ARMop_API - Wrap the new barrier ARMop description text to match the rest of the file
Admin:
  Tested on ARM11, Cortex-A7, -A8, -A9
  Note - relaxed memory consistency rules for Normal vs. Device mean that now more than ever, developers authoring device drivers for ARMv6+ need to be careful to use memory barriers in all the right places (preferably the new ARMop barriers exposed by OS_MMUControl 2 to ensure compatibility with all machines)


Version 5.35, 4.79.2.280. Tagged as 'Kernel-5_35-4_79_2_280'

Detail:
  - Docs/HAL/ARMop_API - Updated with documentation for the new ARMops.
  - s/ARMops - Set up pointers for the new memory barrier ARMops. Add full implementations for ARMv6 & ARMv7; older architectures should be able to get by with a mix of null ops & write buffer drain ops. Update ARMopPtrTable to validate structure against the list in hdr/OSMisc
  - hdr/KernelWS - Reserve workspace for new ARMops. Free up a bit of space by limiting ourselves to 2 cache levels with ARMv7. Remove some unused definitions.
  - hdr/OSMisc - New header defining OS_PlatformFeatures & OS_MMUControl reason codes, OS_PlatformFeatures 0 flags, and OS_MMUControl 2 ARMop indices
  - Makefile - Add export rules for OSMisc header
  - hdr/ARMops, s/ARM600, s/VMSAv6 - Remove CPUFlag_* and MMUCReason_* definitions. Update OS_MMUControl write buffer drain to use DSB_ReadWrite ARMop (which is what most existing write buffer drain implementations have been renamed to).
  - s/GetAll - Get Hdr:OSMisc
  - s/Kernel - Use OS_PlatformFeatures reason code symbols
  - s/vdu/vdudecl - Remove unused definition
Admin:
  Tested on ARM11, Cortex-A8, Cortex-A9


Version 5.35, 4.79.2.279. Tagged as 'Kernel-5_35-4_79_2_279'

Improve support for VMSAv6 cache policies & memory types. Expose raw ARMops via OS_MMUControl & cache information via OS_PlatformFeatures.

Detail:
  Docs/HAL/ARMop_API - Document two new ARMops: Cache_Examine and IMB_List
  hdr/KernelWS - Shuffle workspace round a bit to allow space for the two new ARMops. IOSystemType now deleted (has been deprecated and fixed at 0 for some time)
  s/ARM600 - Cosmetic changes to BangCam to make it clearer what's going on. Add OS_MMUControl 2 (get ARMop) implementation.
  s/ARMops - Switch out different ARMop implementations and XCB tables depending on MMU model - helps reduce assembler warnings and make it clearer what code paths are and aren't possible. Add implementations of the two new ARMops. Simplify ARM_Analyse_Fancy by removing some tests which we know will have certain results. Use CCSIDR constants in ARMv7 ARMops instead of magic numbers. Update XCB table comments, and add a new table for VMSAv6
  s/ChangeDyn - Define constant for the new NCB 'idempotent' cache policy (VMSAv6 normal, non-cacheable memory)
  s/HAL - Use CCSIDR constants instead of magic numbers. Extend RISCOS_MapInIO to allow the TEX bits to be specified.
  s/Kernel - OS_PlatformFeatures 33 (read cache information) implementation (actually, just calls through to an ARMop)
  s/MemInfo - Modify VMSAv6 OS_Memory 0 cache/uncache implementation to use the XCB table instead of modifying L2_C directly. This allows the cacheability to be changed without affecting the memory type - important for e.g. unaligned accesses to work correctly. Implement cache policy support for OS_Memory 13.
  s/Middle - Remove IOSystemType from OS_ReadSysInfo 6.
  s/VMSAv6 - Make sure BangCam uses the XCB table for working out the attributes of temp-uncacheable pages instead of manipulating L2_C directly. Add OS_MMUControl 2 implementation.
  s/AMBControl/memmap - Update VMSAv6 page table pokeing to use XCB table
  s/PMF/osinit - Remove IOSystemType reference, and switch out some pre-HAL code that was trying to use IOSystemType.
Admin:
  Tested on Iyonix, ARM11, Cortex-A7, -A8, -A9, -A15
  Note that contrary to the comments in the source the default NCB policy currently maps to VMSAv6 Device memory type (as per previous kernel versions). This is just a temporary measure, and it will be switched over to Normal, non-cacheable once appropriate memory barriers have been added to the affected IO code.


Version 5.35, 4.79.2.273. Tagged as 'Kernel-5_35-4_79_2_273'

Docs/HAL/NewAPI:
 More clearly describe the intent of the different bus types.
hdr/HALDevice
 Reclassify the OMAP interconnect as either a system bus or peripheral bus, noting their names from the datasheet.
 Reclassify the GPMC as an expansion bus.
 Add AMBA 3's AXI bus, which (along with the existing AHB/APB) is what iMx6 uses. Delete redundant iMx6 interconnect type.

Version 5.35, 4.79.2.249. Tagged as 'Kernel-5_35-4_79_2_249'

The only function to use the abandoned extensions to OS_ClaimDeviceVector, the 2nd & 3rd members of the structure never got used.
Change uses of HAL_IICDevice to pass in the bus, and expect the device number back.
Docs updated accordingly.

Version 5.35, 4.79.2.218. Tagged as 'Kernel-5_35-4_79_2_218'

Detail:
  Docs/RPiNotes - Deleted, contents no longer relevant
  s/HAL, s/Kernel, s/vdu/vduswis, s/pmf/key - Cleaned up debug code
  s/NewIRQs - No need to piggy back on timer 0 IRQ to generate a fake VSync; PushModeInfo already claims/releases TickerV as appropriate if video driver doesn't provide a VSync IRQ.
  s/NewReset - Re-enable LookForHALRTC call, the stack imbalance bug was fixed before the Pi changes were merged in
  s/vdu/vducursoft - Streamline PostWrchCursor a bit by only preserving R14 around RestorePointer if the software pointer is in use
  s/vdu/vdudriver - Amend ModeChangeSub improvements to ensure old external framestore handling logic is used if driver doesn't support framestore growth/realloc
Admin:
  Tested on Raspberry Pi with high processor vectors
  Kernel now looks to be in a good state for merging back into HAL branch
  Note - Software mouse pointer support in vducursoft only checks HALVideoFeatures, so doesn't take into account the capabilities of any GraphicsV driver that may be in use.


Version 5.35, 4.79.2.147.2.20. Tagged as 'Kernel-5_35-4_79_2_147_2_20'

Detail:
  This is a new branch from the current tip of the HAL branch, incorporating
  the changes received from Adrian Lees. The same caveats apply - this is a
  work in progress and will not work on any other platform at present.
Admin:
  Builds, but not tested.

Version 5.35, 4.79.2.147.2.1. Tagged as 'Kernel-5_35-4_79_2_147_2_1'

Detail:
  hdr/ARMops - Reserve OS_PlatformFeatures 0 bit 20 for indicating whether high processor vectors are in use
  s/Kernel - Add local definitions of BYTEWS, LDROSB, STROSB, VDWS macros (previously in Hdr:Macros)
  s/MoreComms - Fix potential buffer overflow when filling error buffer (although GSTrans shouldn't overflow the buffer in the first place?)
  s/Arthur2 - GSRead number detection fix
  s/ArthurSWIs - Updated OS_ReadUnsigned to support reading 64bit numbers
  Docs/ReadUnsigned - Docs for the updated OS_ReadUnsigned interface
Admin:
  Untested!
  Needs HdrSrc 1.86


Version 5.35, 4.79.2.120. Tagged as 'Kernel-5_35-4_79_2_120'

Detail:
 * Stopped calling the broken abort fixup code when running under VMSAv6.
   Might be desirable to update it, possibly farmed out to a separate module -
   still need to think about this.
 * Unaligned load optimisations can now be disabled by the global NoUnaligned
   flag for testing purposes.
 * Extended OS_ReadUnsigned to permit reading of 64-bit unsigned integers.
   See Docs.ReadUnsigned for more details. Also sped it up by using MLA
   (or UMLAL) for most digits rather than repeated addition.
 * Bugfix is OS_GSRead: an uninitialised r0 was being passed to
   OS_ReadUnsigned, causing undesirable effects on rare occasions.
Admin:
  Tested on a rev B7 beagleboard.

Version 5.35, 4.79.2.98.2.8. Tagged as 'Kernel-5_35-4_79_2_98_2_8'

Detail:
  * Added some documentation on previously undocumented HAL calls
  * Corrected NVMemoryFlag_Provision bitmask to match documentation
  * Bugfix: NVMemoryFlag_ProtectAtEnd flag was being ignored
Admin:
  Not tested

Version 5.35, 4.79.2.98. Tagged as 'Kernel-5_35-4_79_2_98'

Detail:
  Updated GraphicsV documentation
  Upped ROM version number - currently matches latest Castle release (5.13)
Admin:
  No code change

Version 5.35, 4.79.2.96. Tagged as 'Kernel-5_35-4_79_2_96'

     1: default ticker based vsync generated whenever no device present to do so
     2: graphicsv handling and spec updated to use the hi 8 bits in the
        reason code (R4) to define the display number. Kernel only knows
        of display 0
Detail:
Admin:
     tested castle  castle added ip


Version 5.35, 4.79.2.81. Tagged as 'Kernel-5_35-4_79_2_81'

  [Not fully implemented - for now leaves at least 16MB free if only
  one RAM area; was 1MB].
* Added HAL_USBControllerInfo, HAL_MonitorLeadID and HAL_Video_Render.
* Added HAL->OS call OS_IICOpV.
* OS_MMUControl now allows independent control of I and C bits.
* Added facility to deactivate keyboard debounce (magic word "NoKd" in
  R2 in KeyV 0).
* Fixed problem with RAM amounts not a multiple of 4MB.
* Supremacy bit (in VDU 19) now sets all 8 bits of supremacy.
* Added PaletteV 14 (reads gamma tables).
* Added Supremacy transfer functions (like gamma correction, but for
  supremacy). Allows easy global supremacy effects in a mode-independent
  fashion. Controlled with PaletteV 15,16.
* Added modes 50-53 (320x240, 1,2,4,8bpp). Intended for small LCD.
* Added 13.5kHz versions of TV modes (selected by Hdr:Machine).
* Upped desktop version to 5.06.

Version 5.35, 4.79.2.66. Tagged as 'Kernel-5_35-4_79_2_66'

OSDelink/RelinkApp now work on the list in opposite orders so that the order of vector claims doesn't get toggled.

Fix for *FX5 not working due to a TST having been swapped for CMP.
Checkprotectionlink option added to HAL version so CMOS lock is
implemented.
Updated HAL docs.

Version 5.35, 4.79.2.58. Tagged as 'Kernel-5_35-4_79_2_58'

Detail:
  * Merged in the change to RISC OS 4.02 kernel that moved the GSTrans
    workspace out of scratch space.
  * Fixed a few bugs in callback postponement, and interrupt holes in
    callback dispatch. See Docs.CallbackChange for full info.
  * Fixed SystemSizeCMOS to SysHeapCMOS - wouldn't build as was.
  * Added an export of a C version of Hdr:HALDevice, based on the Hdr2H
    translation but with an additional struct definition. Required by
    SoundControl 1.00.
  * Added some additional location and ID allocations to Hdr:HALDevice.
    Required by today's HAL and SoundControl.
Admin:
  Partially tested.

Version 5.35, 4.79.2.56. Tagged as 'Kernel-5_35-4_79_2_56'

*Configure ANYTHINGsize was broken due to not setting R0 to ReadUnsigned
IIC ack message uninternationalised
OS_Memory was saying we only had 4M of RAM
VDU4 scrolling when output was switched to sprite was causing corruption
on use of CTRL-J and CTRL-K
Default SystemSize CMOS set to 32k

Version 5.35, 4.79.2.55. Tagged as 'Kernel-5_35-4_79_2_55'

Added 256-colour version of the (high-resolution only) teletext code, and support for teletext when hardware scroll is disabled. Both are required for Tungsten.

Turned off the module init/final service calls, since we still don't have an
allocation for them.
Upped the OS version number to 5.01.

Version 5.35, 4.79.2.53. Tagged as 'Kernel-5_35-4_79_2_53'

Detail:
  * Rejigged documented meaning of device "Location" field so that we can
    fit full PCI locations in.
  * Defined lots of device "Type" values in Hdr:HALDevice.
  * Removed obsolete DMA-related HAL entries in Hdr:HALEntries (no longer
    required by DMAManager 0_15-4_4_2_6, no longer provided by Tungsten HAL
    0.07).
  * OS_Hardware 2 and 3 actually work now.
  * Changed OS_Hardware 4 to take a maximum major version number to match.
  * HAL workspace is now USR mode readable.
  * Service calls issued after module initialisation/finalisation (see
    Docs.ModPostServ).
Admin:
  OS_Hardware tested, service calls not tested.

Version 5.35, 4.79.2.52. Tagged as 'Kernel-5_35-4_79_2_52'

Detail:
  Lots of changes since last version, at least the following:
  * Updated OS timestamp, removed alpha status
  * Negative INKEY OS version changed to &AA
  * GraphicsV is now alocated vector number &2A
  * ROM moved up to &FC000000
  * Max application slot increased to 512 Mbytes (for now)
  * Max size of RMA increased to 256 Mbytes
  * RMA is now first-created dynamic area (so it gets lowest address after
    top of application slot)
  * OS_Memory 10 reimplemeted
  * New OS_ReadSysInfo 6 values 18-22 added
  * OS_ReadSysInfo 8 gains flag bit to indicate soft power-off
  * Misc internal top-bit-set-address fixes
  * *ChangeDynamicArea can take sizes in megabytes or gigabytes
  * Magic word "&off" in R0 passed to OS_Reset powers down if possible
  * Added acceleration: block copy; CLS; text window scroll up; rectangle
    fill
  * Disabled LED flashing in page mode (liable to crash)
  * Masked sprite plot and VDU 5 text avoids reading the screen if possible
  * Framestore made USR mode accessible
  * Fix for VDU 5,127 bug - now relies on font definitions being in extreme
    quarters of memory, rather than bottom half
  * Allocated 64-bit OS_Convert... SWIs
  * IIC errors use allocated error numbers
  * Looks for Dallas RTC before Philips RTC because we're using a Philips
    NVRAM device with the same ID
  * Fix to bug that meant the oscillator in the Dallas RTC wasn't enabled
  * Default mouse type (USB) changed to allocated number
  * Ram disc max size increased to 128 Mbytes (Ursula merge) and made
    cacheable for StrongARMs (not XScale)
  * Branch through zero handler now works in USR mode, by use of a
    trampoline in the system stack to allow PC-relative register storage
  * Address exception handler changed to not use 0 as workspace
  * OS_Memory 13 extended to allow specification of cacheability and access
    privileges
  * Added OS_Memory 16 to return important memory addresses
  * RISCOS_MapInIO() takes cacheable flag in bit 3, access permissions in
    bits 10 and 11, doubly-mapped flag in bit 20, and access permissions
    specified flag in bit 21
  * Bug fix in last version for application abort handlers didn't quite
    work; register shuffle required
  * "Module is not 32-bit compatible" error now reports the module name
  * Default configured language changed from 10 to 11 (now Desktop again)

Version 5.35, 4.79.2.51. Tagged as 'Kernel-5_35-4_79_2_51'

Detail:
  * Implemented OS_Hardware 2, 3 and 4 as described in Docs.HAL.NewAPI.
  * Added new OS->HAL and HAL->OS routines to register HAL devices with the
    OS during hard resets.
  * Updated Docs.HAL.NewAPI to correct inconsistencies, fill in missing
    definitions, and allow for interrupt sharing.
  * Now uses OS_LeaveOS to trigger callbacks after ROM module init.
Admin:
  Untested. Requires new HAL.

Version 5.35, 4.79.2.49. Tagged as 'Kernel-5_35-4_79_2_49'

Version 5.35, 4.79.2.48. Tagged as 'Kernel-5_35-4_79_2_48'

Version 5.47. Tagged as 'Kernel-5_47'

Look for LongCommandLine flag, command line size currently
set at 1k.
For HAL/32bit builds, the kernel buffer space is at high
(top bit set) address, which may break some code using signed
comparisons. So *beware* that there may be some latent
bugs in old kernel code using these buffers, not yet found.
One such bug, in s.Arthur2 found and fixed.
Tested moderately on ARM9 desktop build.
Lovely to reimplement things I did two and half years ago.

Version 5.35, 4.79.2.37. Tagged as 'Kernel-5_35-4_79_2_37'

* Allows HAL-driven software resets.
* Sound buffers corrected to be uncacheable.

Version 5.35, 4.79.2.33. Tagged as 'Kernel-5_35-4_79_2_33'

Version 5.35, 4.79.2.18. Tagged as 'Kernel-5_35-4_79_2_18'

Version 5.35, 4.79.2.17. Tagged as 'Kernel-5_35-4_79_2_17'

kernel now attempts to substitute video mode numbers in face of h/w with limited bits-per-pixel support (not tested yet)

HAL_API document added - early draft only, of interest to those
writing or modifying HALs for new h/w
ARMop_API document added - early draft only, of interest only
to those modifying kernel to support new ARM cores
*** polite comments on HAL_API welcome ***

Version 5.35, 4.79.2.15. Tagged as 'Kernel-5_35-4_79_2_15'

First attempt at ARM9 support, and general clean-up of old ARM-specific code, now using vectored ARMops.

Not tested.

Version 5.35, 4.79.2.14. Tagged as 'Kernel-5_35-4_79_2_14'

More stuff. Up to the desktop now; cache on, working keyboard. Some source restructuring to start to make splitting it up into several object files more feasible.

Detail:
  Added the HAL NVRAM entries.
  Modified i2cutils to use the HAL entries for NVRAM and behave sensibly if the HAL reports that there is no NVRAM, in which case there must be a forced reset_cmos call so that the cache gets set up sensibly.
Admin:
  Tested under the RPC emulator and appears to be working correctly, although some calls to IIC are still being made in the no nvram case.

Version 5.35, 4.79.2.8. Tagged as 'Kernel-5_35-4_79_2_8'

Version 5.35, 4.79.2.5. Tagged as 'Kernel-5_35-4_79_2_5'

Version 5.35, 4.79.2.2. Tagged as 'Kernel-5_35-4_79_2_2'

* Added ARM_IMB and ARM_IMBRange SWIs as recommended by ARMv5.
* Some early prototype HAL bits popped in - a lot of source restructuring still
  to come.
* New debug target creates an AIF image with debug information, and translates
  this into an ASCII object file for the 16702B logic analyser.

Version 5.35, 4.79.2.1. Tagged as 'Kernel-5_35-4_79_2_1'

Added compile-time support for full-resolution teletext characters in teletext emulation mode (MODE 7) for that authentic BBC Micro feel.

  Also introduced a few useful teletext control features via VDU 23,18.
  Unrelatedly, fixed *ScreenLoad to work for interlaced displays.

Detail:
  The new typeface is designed on a 16x20 grid (previously we had used 8x10),
  so it uses a screen resolution of 640x500 pixels (rather than 320x250).
  Since we have been unable to source a genuine teletext font, and since
  examination of a BBC Micro suggests that the genuine font may not have been
  a power-of-2 pixels wide, I have designed one specially, based upon the one
  supplied in Zap distributions (a 12x20 font). Rather than increase the
  amount of workspace that the kernel requires for cacheing graphic
  characters, it now generates them on the fly, as they are required; this
  should only add about 25% to their rendering time.

  The new VDU 23 sequences are as follows:

  VDU 23,18,0,mode,0,0,0,0,0,0
    Switch transparency mode
      mode = 0: "Text" mode: the whole display is set opaque
      mode = 1: "Mix" mode: foreground colours, and both foreground and
        background of boxed text are opaque; non-boxed background colours are
        all transparent
      mode = 2: "Box" mode: boxed regions are opaque, others are transparent
      mode = 3: "TV" mode: the whole display is set transparent
    Default is mode = 0.

  VDU 23,18,1,suspend,0,0,0,0,0,0
    Suspend or resume bitmap updates
    This call allows an application to request that the kernel suspends
    updates to the framebuffer bitmap. This allows for a significant speed
    increase in the rendering time for a large amount of text, for example
    when redrawing a complete teletext page, because each time you plot a
    single character, it can cause the whole of the rest of the line to be
    re-rendered. When you switch out of suspend mode, the whole screen is
    refreshed in a single pass. Note that the appearance of the display is
    undefined is you cause a hardware scroll while in suspend mode.
      suspend = 0: screen update is enabled
      suspend = 1: screen update is suspended
    Default is suspend = 0.

  VDU 23,18,2,reveal,0,0,0,0,0,0
    Reveal/conceal
      reveal = 0: characters between the Conceal control code and the next
        colour control code are replaced by spaces
      reveal = 1: all characters are displayed
    Default is reveal = 0.

  VDU 23,18,3,black_emable,0,0,0,0,0,0
    Enable/disable black foreground colour control codes
      black_enable = 0: control codes &80 and &90 do nothing
      black_enable = 1: control code &80 selects black text, control code
        &90 selects black graphics
    Default is black_enable = 0.

  I have performed some timing tests on the rendering of complete teletext
  pages grabbed from the teletext server. These show that the new code
  generally imposes a 2x speed hit. However, when using the VDU 23,18,1
  suspend function, this improves to a 20% speed increase when compared to
  the old low-resolution code. Better still, because the framebuffer is only
  being updated for the final stage of this process, the screen *appears* to
  be updated some 3x faster than with the old code!

  A comment on the VDU variable Log2BPC is in order: in previous kernels,
  this was able unambiguously to refer to both the framebuffer width of a
  character in bytes, and the framebuffer width of an "addressable pixel" in
  bits; this no longer works with the 16-pixel wide teletext font. Bearing
  in mind that future kernels may support Unicode system fonts where the
  width varies from character to character, I have chosen to fix Log2BPC to
  the "addressable pixel" definition.

Admin:
  Requires HdrSrc 0.89 and (for non-desktop builds) Interlace 0.61. A monitor
  definition file containing a definition for a 640x500 screen mode is also
  required; version 0.40 of ModeFiles contains a suitable mode for STB-400.

  Tested fairly rigourously on an Ursula build, a Lazarus build and an
  STB-400 build, using genuine teletext pages and Yellow River Kingdom.

Version 5.30. Tagged as 'Kernel-5_30'