Amstrad Whole Memory Guide - The machine pack

From CPCWiki - THE Amstrad CPC encyclopedia!
Revision as of 14:00, 25 September 2019 by 2f1cafc (Talk | contribs) (ocr'd page 19)

Jump to: navigation, search

Chapter 3 in the book Amstrad Whole Memory Guide.


Broadly speaking, the Machine Pack is reponsible for the control of hardware peripherals, but it will be convenient to include the main initialisation processes under this heading, since they are largely concerned with peripheral setting-up.

Several of the Machine Pack routines depend on the action of other routines to set up data. To understand this data in full, you need to read 'The Ins and Outs of the AMSTRAD CPC464', which gives full details of the peripheral codes. Only the more essential codes will be defined here.

Main Reset

At switch—on, or in response to instruction code &C7, location 0000 is entered. At switch-on, lower ROM is enabled, but the ROM routines are later copied to the corresponding RAM locations in this area, so the enable state of the lower ROM is then unimportant. However, the first action of the reset routine is to output &89 to the Video Gate array on I/0 address 7FXX, and this enables lower ROM, disables upper ROM, and also sets up Mode 1. There being no further room in the RST Area the routine jumps to 0580 to continue reset action.

Interrupt is disabled, and &82 is output to F7XX. This sets the PPI (Parallel Peripheral Interface) to output on ports A and C, input on port B. Zero outputs to F4XX and F6XX clear ports A and C, while an output of &7F to EFXX initialises the printer port. Bit 7 is low, the other bits are high.

The CRT Controller is then set up. There are two alternative sets of values for this, one for 50 Hz frame scan and the other for 60 Hz. The set to be used is determined by reading port B, bit 4. If this bit is true, 50 Hz values are used, while the 60 Hz values are used if the bit is false, this being determined by the presence of Link 4 on the main printed circuit board.

Scanned pages