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Article: Single gate enhances 68000-family systems. (integrated circuit) (Tutorial)
- Article from:
- EDN
- Article date:
- December 9, 1993
- Author:
CopyrightCOPYRIGHT 1993 Reed Business Information, Inc. (US). This material is published under license from the publisher through the Gale Group, Farmington Hills, Michigan. All inquiries regarding rights should be directed to the Gale Group. (Hide copyright information)
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The circuit in Fig 1 employs a 68008 along with a single 27C256 ROM for the resident program and a battery-backed DS1225 RAM for flags, scratchpad, etc. Placing the system RAM in the Motorola [mu]P's zero page and carefully writing your ROMmable code can save enough bytes of program memory to increase the effective size of the system ROM.
The 68000 family's instruction set differentiates between a page-zero address and an address in any another 32-kbyte page of memory. Further, if the addressing mode that a given instruction uses is an absolute address, such as the address of a flag in RAM, it attaches to the instruction as an extension word. The length of this ...